Blood – HISTOLOGY

🔹 Red Pulp of the Spleen:

The spleen is divided into two main components:

• White pulp: lymphoid tissue mainly involved in immune response.

• Red pulp: involved in filtering blood, removing old/damaged red blood cells, and storing blood elements.

The red pulp contains:

• Cords of Billroth (also called splenic cords): These are reticular connective tissue strands filled with macrophages, red blood cells, lymphocytes, plasma cells, and granulocytes.

• Sinusoids: Specialized vascular channels lined by endothelial cells through which blood flows slowly, allowing phagocytosis of damaged cells.

🔹 Structure-Function:

• Cords of Billroth trap old or damaged erythrocytes, which macrophages then phagocytose.

• This process helps maintain healthy blood by clearing senescent cells.

❌ Why the Other Options Are Incorrect:

1. Lymphocytes

   • Found predominantly in the white pulp, particularly in the follicles and PALS.

   • ❌ Not the main component of red pulp.

2. Central arteriole

   • These arteries run through the white pulp, surrounded by PALS.

   • ❌ Located in white pulp, not red pulp.

3. None of these

   • Incorrect because cords of Billroth are the hallmark of red pulp.

4. PALS (Periarteriolar lymphoid sheaths)

   • T-cell rich zones in the white pulp surrounding central arterioles.

   • ❌ Part of white pulp, not red pulp.

🧬 Immunoglobulin G (IgG)

• IgG is the most abundant antibody isotype in serum, comprising ~75–80% of the total serum immunoglobulins.

• It plays a key role in long-term immunity and immunologic memory.

🔑 Key Functions of IgG:

1. Crosses the placenta — provides passive immunity to the fetus.

2. Neutralizes toxins and viruses.

3. Opsonization — enhances phagocytosis by marking pathogens.

4. Activates the classical complement pathway.

5. Present in extracellular fluids, lymph, and blood.

Its monomeric structure allows it to easily diffuse into tissues, making it ideal for systemic defense.

❌ Why the Other Options Are Incorrect:

1. IgA

• Most abundant overall in the body (including mucosal secretions), but not in serum.

• In serum, it makes up only ~10–15%.

• Predominant in mucosal secretions (e.g., saliva, tears, milk).

• ❌ Not the most abundant in serum.

2. IgD

• Trace amounts in serum.

• Functions mainly as a B-cell receptor on naïve B cells.

• ❌ Extremely low serum levels.

3. IgE

• Involved in allergic reactions and parasite defense.

• Binds tightly to mast cells and basophils.

• Found in very low concentrations in serum.

• ❌ Least abundant in serum.

4. IgM

• First antibody produced in a primary immune response.

• Exists as a pentamer in serum.

• Makes up about 5–10% of serum immunoglobulins.

• ❌ Not the most abundant.

🧬 Immunoglobulin M (IgM)

• IgM is the first antibody produced in a primary immune response.

• In its secreted form, IgM exists as a pentamer:

  • Composed of five antibody monomers.

  • These monomers are linked by disulfide bonds and a J (joining) chain.

🔑 Key Features of IgM:

• Pentameric structure = 10 antigen-binding sites

• Very efficient at agglutination and complement activation.

• Found predominantly in the intravascular compartment (blood and lymph), as it’s too large to diffuse easily into tissues.

• Because of its multivalency, it’s very effective at neutralizing pathogens early in infection.

❌ Why the Other Options Are Incorrect:

1. IgE

• Involved in allergic reactions and parasitic defense.

• Monomeric structure.

• Binds to Fc receptors on mast cells and basophils.

• ❌ Not pentameric.

2. IgD

• Found mainly on naïve B cells as an antigen receptor.

• Plays a role in B cell activation.

• Monomeric, very low concentration in serum.

• ❌ Not pentameric.

3. IgG

• The most abundant antibody in serum.

• Can cross the placenta, activates complement, opsonizes pathogens.

• Monomeric structure.

• ❌ Not pentameric.

4. IgA

• Exists in two forms:

  • Monomeric in serum.

  • Dimeric in secretions (saliva, tears, mucus, milk) with a J chain.

• ❌ Dimeric, not pentameric.

✳️ Immunoglobulin A (IgA)

IgA plays a crucial role in mucosal immunity. It exists in two forms, depending on where it is found in the body:

1. Serum IgA – Monomeric (like IgG).

2. Secretory IgA (sIgA) – Dimeric, held together by a J (joining) chain and protected by a secretory component.

🔗 What Is the J Chain?

• The J chain is a polypeptide that links two monomeric IgA units into a dimer.

• It also facilitates the transport of IgA across epithelial cells via the poly-Ig receptor, leading to secretion into mucosal fluids like:

  • Saliva

  • Tears

  • Mucus

  • Colostrum

This dimeric structure, along with the secretory component, protects IgA from enzymatic digestion, making it ideal for function in hostile environments like the gut or respiratory tract.

❌ Why the Other Options Are Incorrect:

1. IgD

• Found primarily as a receptor on naïve B cells.

• Monomeric structure.

• 🛑 Does not form dimers and does not have a J chain.

2. IgM

• 🟡 Trick option — IgM does have a J chain, but it forms a pentamer, not a dimer.

• The J chain helps assemble its five monomer units.

• So, while IgM has a J chain, it is not dimeric — it is pentameric.

• ❌ Not the correct answer based on the dimeric + J chain description.

3. IgE

• Involved in allergies and parasitic responses.

• Monomeric, no J chain.

• Binds to Fc receptors on mast cells and basophils.

• 🛑 No role in mucosal secretions or dimer formation.

4. IgG

• Main antibody in blood and extracellular fluid.

• Monomeric, crosses the placenta.

• ❌ No J chain, no dimerization.

🩸 What is Erythroblastosis Fetalis?

Also known as Hemolytic Disease of the Newborn (HDN), this condition arises when:

• An Rh-negative mother is exposed to Rh-positive fetal red blood cells, usually during delivery of her first Rh-positive baby.

• Her immune system then creates anti-Rh antibodies (specifically IgG antibodies) against the Rh antigen.

• In subsequent pregnancies, if the fetus is again Rh-positive, the maternal IgG antibodies can cross the placenta and attack fetal red blood cells, leading to hemolysis and increased erythroblasts in fetal circulation — hence the name “erythroblastosis fetalis.”

🌉 Why IgG Is the Correct Answer:

• IgG is the ONLY class of antibody that can cross the placenta.

• This is due to the neonatal Fc receptor (FcRn) expressed on placental cells, which specifically transports IgG from maternal to fetal circulation.

• This transfer is protective in most cases, giving passive immunity to the fetus.

• However, in conditions like HDN, it can lead to pathology due to the mother’s immune response against fetal antigens.

❌ Why the Other Options Are Incorrect:

1. IgM

• Largest antibody (pentamer) → too large to cross the placenta.

• First antibody produced in an immune response.

• Important in blood group reactions, but does not participate in HDN because it cannot cross the placental barrier.

2. IgE

• Involved in allergic reactions and defense against parasites.

• Binds to mast cells and basophils, does not cross the placenta.

3. IgA

• Found mainly in secretions (tears, saliva, mucosa, breast milk).

• Exists in dimeric form with a secretory component.

• Does not cross the placenta.

4. IgD

• Functions mainly as a B-cell receptor.

• Very low levels in serum.

• No significant role in fetal immunity.

• Does not cross the placenta.

🧠 Hint to Think Critically:

Consider which immunoglobulin is selectively transported to the fetus, providing both protection and, in rare cases, pathology. What feature allows only one class of antibody to reach fetal circulation while others cannot?

✳️ Immunoglobulin A (IgA):

IgA is the principal antibody found in mucosal secretions — making it essential for mucosal immunity.

It’s most abundant in areas where the body interfaces with the external environment:

• Respiratory tract

• Gastrointestinal tract

• Urogenital tract

• Saliva

• Tears

• Breast milk (especially colostrum)

🧬 Key Features of IgA:

• Exists in two forms:

  • Monomeric IgA in serum.

  • Dimeric IgA in secretions, joined by a J chain and associated with a secretory component that protects it from enzymatic degradation.

• Provides local immunity by neutralizing pathogens before they invade deeper tissues.

• Does not fix complement via the classical pathway — this prevents inflammatory damage at delicate mucosal surfaces.

❌ Why the Other Options Are Incorrect:

1. IgM

• First antibody produced in a primary immune response.

• Predominantly found in the blood and lymph.

• It’s pentameric, effective in complement activation and agglutination.

• 🛑 Not typically found in mucosal surfaces or secretions.

2. IgD

• Found in very low concentrations in serum.

• Mainly acts as a receptor on naïve B cells.

• 🛑 Has no major role in mucosal immunity or secretions.

3. IgE

• Involved in allergic reactions and defense against parasites.

• Binds to mast cells and basophils, triggering histamine release.

• Found in tissues but not in secretions like tears or saliva.

• 🛑 Not related to mucosal or secretory defense.

4. IgG

• Most abundant antibody in blood and extracellular fluid.

• Can cross the placenta, providing passive immunity to the fetus.

• Important in long-term immunity and opsonization.

• 🛑 Not typically found in external secretions like tears or milk.

Blood as a Connective Tissue:

Blood is indeed a specialized connective tissue, even though it differs from most other connective tissues in some key ways. Let’s understand how it fits the criteria of connective tissue:

• Origin: Blood is mesodermal in origin, like all connective tissues.

• Components: All connective tissues consist of:

  1. Cells

  2. Fibers

  3. Ground substance

These components together form the extracellular matrix (ECM). Now, let’s analyze each of these components specifically in blood.

🔬 1. Ground Substance in Blood:

• In typical connective tissue, the ground substance is a gel-like material (like in cartilage or bone).

• In blood, the ground substance is plasma – which is fluid in nature.

• Plasma is rich in water, electrolytes, proteins (like albumin, globulins, fibrinogen), and other solutes.

• This is what makes blood’s ECM fluid, distinguishing it from other connective tissues.

✅ Hence, the statement “Its ground substance is of a fluid nature” is correct.

❌ Why the Other Options Are Incorrect:

1. “It contains fibers only”

🔻 Incorrect because:

• Blood doesn’t contain visible fibers under normal conditions.

• Fibrin fibers (from fibrinogen) appear only during clotting, not normally.

• Also, this option ignores cells (e.g., RBCs, WBCs, platelets), which are abundantly present.

2. “It contains both cells and fibers”

🔻 Also incorrect, though it sounds appealing:

• Blood contains cells, yes (erythrocytes, leukocytes, thrombocytes).

• However, fibers are not normally present unless the blood clots.

• So, under normal (non-clotted) conditions, no actual fibers are present—only soluble proteins like fibrinogen.

3. “It has a greater proportion of cells as compared to fibers”

🔻 Misleading:

• Again, fibers aren’t present in normal blood.

• So, while the idea of “more cells than fibers” might seem true, it’s conceptually wrong because we don’t compare cells to fibers that aren’t normally there.

4. “None of these”

🔻 Incorrect because one of the options is indeed correct — the one about the fluid ground substance.

✅ Correct Answer: Red pulp

Why?
The red pulp of the spleen is the region responsible for filtering and removing old, damaged, or abnormal RBCs.

• It contains:

  • Splenic sinusoids — leaky vascular channels lined by specialized endothelial cells

  • Cords of Billroth — filled with macrophages, reticular cells, RBCs, and plasma cells

In this microenvironment, RBCs are squeezed through narrow slits. Old or inflexible RBCs fail to pass through and are phagocytosed by macrophages, where hemoglobin is broken down into iron, globin, and bilirubin.

This is the spleen’s “quality control zone” for red cells.

❌ Why the other options are incorrect:

 White pulp

• Involved in immune responses. Contains lymphocytes and germinal centers, not RBC clearance.

 Splenic module

• Not an anatomical term — likely a distractor or incorrect labeling. No known structure by this name.

 Malpighian corpuscle

• Refers to lymphoid nodules within white pulp (not to be confused with renal Malpighian corpuscles).

• These are immune structures, not sites of RBC breakdown.

 PALS (Periarteriolar lymphoid sheaths)

• A T-cell–rich zone of the white pulp, surrounding central arterioles.

• Not involved in RBC clearance or filtration.

✅ Correct Answer:  An immature erythrocyte

Why?
Reticulocytes are immature red blood cells (RBCs) that have just been released from the bone marrow into the bloodstream.

• They still contain residual ribosomal RNA, which can be seen with supravital stains (like methylene blue) as a reticular network — hence the name “reticulocyte.”

• After ~1–2 days in the circulation, they mature into fully functional erythrocytes.

• An increased reticulocyte count reflects a healthy bone marrow response to anemia or blood loss — it’s the body’s way of compensating.

❌ Why the other options are incorrect:

 An erythrocyte

• Not quite. Reticulocytes become erythrocytes but are still immature and structurally different from mature RBCs.

 An agranulocyte

• Agranulocytes refer to white blood cells like lymphocytes and monocytes, not red cells.

 A granulocyte

• Granulocytes are neutrophils, eosinophils, and basophils — again, these are leukocytes, not erythroid lineage cells.

 A mature erythrocyte

• Incorrect. Reticulocytes are not fully mature — they lack a nucleus but still contain residual organelles, which are lost during maturation.

✅ Correct Answer:  The thymus lacks afferent lymphatics

Why?
The thymus is a primary lymphoid organ responsible for the maturation of T lymphocytes. It does not receive lymph from tissues, because its job is to train T cells, not to filter or respond to antigens coming from peripheral tissues.

• Therefore, it lacks afferent lymphatic vessels, which are the vessels that bring lymph into an organ (a hallmark of secondary lymphoid organs, like lymph nodes).

• The thymus does have efferent lymphatics, which drain mature T cells out of the thymus.

This is a key distinguishing feature from lymph nodes, which:

• Are secondary lymphoid organs

• Have both afferent and efferent lymphatic vessels

• Filter lymph and initiate immune responses to antigens carried in from tissues

❌ Why the other options are incorrect:

The thymus is a secondary lymphoid organ

• Incorrect. The thymus is a primary lymphoid organ. It does not participate in antigen-dependent immune responses, unlike lymph nodes or spleen.

 The thymus lacks efferent lymphatics

• Incorrect. The thymus does have efferent lymphatic vessels that carry away mature T cells to the circulation.

 The thymus is derived from the mesoderm only

• Partially incorrect. The epithelial component of the thymus is derived from endoderm (third pharyngeal pouch), and the mesenchymal component (connective tissue, vasculature) comes from mesoderm — so it’s not solely mesodermal.

 The thymus is derived from the endoderm only

• Also partially incorrect. While the functional epithelium is from endoderm, mesoderm contributes to supporting stroma and vasculature.

✅ Correct Answer:  They are composed of epithelial reticular cells type VI

Why?
The whorl-like corpuscles described in the thymus are known as Hassall’s corpuscles, which are a hallmark of the medulla of the thymus.

• Composed of type VI epithelial reticular cells, these structures form concentrically arranged keratinized layers.

• They are involved in educating T cells and in the induction of central tolerance, possibly through interactions with dendritic cells.

• Hassall’s corpuscles are unique to the thymus and are diagnostic of it on histological sections.

❌ Why the other options are incorrect:

They have their embryological origin from mesoderm

• Incorrect. Thymic epithelial cells (including those in Hassall’s corpuscles) are derived from endoderm, specifically from the third pharyngeal pouch.

 They are derived from T memory cells

• Incorrect. These are stromal structures, not lymphocytes.

• Hassall’s corpuscles are composed of epithelial reticular cells, not T cells.

They are frequently seen in thymus of young individuals

• Partially misleading. While present in children, Hassall’s corpuscles become more prominent with age, especially during thymic involution.

 They are mainly located in the thymic cortex

• Incorrect. Hassall’s corpuscles are found in the medulla of the thymus, not the cortex, which is densely packed with immature T cells.

✅ Correct Answer:  Cords of Billroth

Why?
The spleen is divided into red pulp and white pulp, each with distinct functions:

• Red pulp is responsible for filtering the blood, especially removing old or damaged red blood cells and platelets.
  It contains:

  • Splenic sinusoids (venous channels)

  • Cords of Billroth — networks of reticular connective tissue, macrophages, plasma cells, and RBCs

• The Cords of Billroth trap old or defective RBCs so that they can be phagocytosed by macrophages.

❌ Why the other options are incorrect:

Lymphocytes

• Primarily found in the white pulp, especially in the PALS and lymphoid follicles.

• While some may be present in red pulp, they are not a defining feature.

 Central arteriole

• Found in the white pulp, particularly at the center of the PALS.

• Not in the red pulp.

None of these

• Incorrect because Cords of Billroth are found in the red pulp.

 PALS (periarteriolar lymphoid sheaths)

• Part of white pulp, surrounding the central arteriole.

• Rich in T lymphocytes, not associated with red pulp.

✅ Correct Answer:  Pharyngeal tonsil

Why?
The pharyngeal tonsil, located in the nasopharynx (posterior wall), is part of Waldeyer’s ring, which consists of lymphoid tissues guarding the upper respiratory and digestive tracts.

Unlike the palatine and lingual tonsils, which are exposed to the oropharynx and are thus lined by non-keratinized stratified squamous epithelium (to withstand mechanical stress from food), the pharyngeal tonsil is exposed to air and particulates, so it is lined by ciliated pseudostratified columnar epithelium (also known as respiratory epithelium).

This epithelium is:

• Ciliated, to trap and move particles

• Pseudostratified, appearing layered due to nuclei at different levels, but all cells contact the basement membrane

❌ Why the other options are incorrect:

 Lymph nodes

• Not covered by any epithelium — they are internal lymphoid organs surrounded by a capsule of connective tissue.

• No lining epithelium is involved.

 Lingual tonsil

• Located at the posterior tongue, and lined by non-keratinized stratified squamous epithelium.

• This is because of constant mechanical stress and exposure to food.

 Spleen

• Like lymph nodes, it’s an internal organ — not covered by any epithelial lining, but by a fibrous capsule.

 Palatine tonsil

• Situated between the palatoglossal and palatopharyngeal arches.

• Lined by non-keratinized stratified squamous epithelium, due to exposure to mechanical friction (swallowing, chewing, etc.)

• Hassall’s (thymic) corpuscles are distinctive, concentric, whorl-like structures found only in the thymic medulla.

• They are formed by type VI epithelial reticular cells—specialized thymic epithelial cells that keratinize centrally as they mature.

❌ Why the Other Options Are Incorrect:

• They are mainly located in the thymic cortex:
  Hassall’s corpuscles reside in the medulla, not the cortex.

• They have their embryological origin from mesoderm:
  Thymic epithelium (and thus these corpuscles) derives from endoderm of the third pharyngeal pouch.

• They are derived from T memory cells:
  Corpuscles are epithelial in origin, not lymphoid or thymocyte-derived.

• They are frequently seen in thymus of young individuals:
  Actually, they increase in number and size with age, being more abundant in older thymus.

Lymphatic tissues can be encapsulated (e.g., lymph nodes, spleen, thymus) or unencapsulated. Peyer patches are clusters of lymphoid follicles located in the ileum of the small intestine. They:

• Lack a true connective tissue capsule.

• Contain specialized M cells that transport antigens from the intestinal lumen to underlying immune cells.

• Serve as a key component of gut-associated lymphoid tissue (GALT).

❌ Why the Other Options Are Incorrect:

• Spleen:
  Completely encapsulated by dense connective tissue.

• Tonsils:
  Partially encapsulated; have deep crypts but are covered by a partial capsule.

• Lymph node:
  Fully encapsulated; follicles lie within a fibrous capsule.

• Thymus:
  Enveloped by a connective tissue capsule, with trabeculae extending inward.

Connective tissues are characterized by cells embedded in an extracellular matrix composed of fibers and ground substance. Blood is considered a special connective tissue because:

• Its ground substance is the plasma, a fluid medium rich in proteins, nutrients, and dissolved solutes.

• Although blood contains fibers (fibrinogen) and cells (red cells, white cells, platelets), the defining feature is the liquid nature of its matrix, allowing it to transport substances throughout the body.

❌ Why the Other Options Are Incorrect:

• It consists of only fibers:
  Blood contains fibers only upon clotting; normally, fibrinogen is soluble until activated.

• It has a greater ratio of cells than fibers:
  While cell-rich, this doesn’t explain its connective tissue classification; many tissues have more cells than fibers.

• It consists of hydroxyapatite crystals in its matrix:
  That describes bone, not blood.

• It is composed of cells and fibers:
  Although true, this is insufficient—many tissues share that composition. The fluid ground substance is the key distinguishing feature.

This question is asking about staining characteristics of cells and their components in histological studies. Different cellular structures absorb different stains depending on their chemical properties, allowing us to differentiate cell types and organelles under a microscope.

Let’s focus on the term that is correct and uniquely used in histology:

🧪 Azurophilia:

• This refers to the purple staining of lysosomal granules (also called primary granules) found in certain white blood cells, particularly neutrophils and monocytes.

• These granules contain enzymes like myeloperoxidase, acid hydrolase, and defensins.

• The purple color results from Romanowsky-type stains (e.g., Wright or Giemsa), which cause these granules to take on a bluish-purple (azurophilic) hue.

• Azurophilia is a distinct term used in hematology and histology, especially in peripheral blood smears and bone marrow examination.

✅ Correct Answer:

Azurophilia (purple stain) for lysosome of WBCs

❌ Why the Other Options Are Incorrect:

• Neutrophilia (pink stain) for neutrophil granules:
  This is a clinical term, not a stain. Neutrophilia refers to increased neutrophil count, not a staining pattern. Neutrophil granules do stain, but we don’t call it “neutrophilia” in histology.

• Basophilia (blue stain) for DNA in ribosomes:
  Ribosomes contain RNA, not DNA. Also, basophilia typically refers to the nucleus and rough ER (RNA-rich areas) staining blue with hematoxylin, but the DNA in the nucleus, not ribosomes, is the main basophilic structure.

• Hemophilia (red stain) for hemoglobin:
  “Hemophilia” is a genetic bleeding disorder, not a stain. The name is unrelated to staining properties and thus completely incorrect in this context.

• Eosinophilia (yellow stain) for hemoglobin:
  Eosinophilia is the term for eosin-staining structures, but eosin stains pink, not yellow. Hemoglobin is indeed eosinophilic, but the color described is inaccurate.

Fibroblasts are the principal cells involved in wound healing and fibrosis. They are responsible for producing collagen, elastin, fibronectin, and other extracellular matrix components necessary to rebuild damaged tissue.

Origin:

• In the context of tissue injury, fibroblasts are derived from local mesenchymal cells.

• These mesenchymal stem-like cells reside in connective tissue and can differentiate into multiple lineages, including fibroblasts, adipocytes, osteoblasts, and chondrocytes.

In wound healing:

• Fibroblasts migrate to the site of injury in response to signals like PDGF, TGF-β, and FGF.

• They lay down a collagen-rich matrix to form a scaffold for tissue regeneration.

• Eventually, some may become myofibroblasts, helping with wound contraction.

Why the correct answer is right:

• Local mesenchyme provides the progenitor cells that become fibroblasts, critical for extracellular matrix production and tissue repair.

Why the other options are incorrect:

• Platelets:
  Important for clot formation and release of growth factors, but they do not differentiate into fibroblasts.

• Endothelium:
  These are vascular lining cells, not progenitors of fibroblasts. They may contribute to angiogenesis but not collagen production.

• Vascular fibrosis:
  This is a pathological outcome, not a cellular origin.

• Epithelium:
  These cells line surfaces and organs. They don’t give rise to fibroblasts, though epithelial–mesenchymal transition (EMT) may occur under certain pathological conditions—not typical for wound healing.

The palatine tonsil is part of the mucosa-associated lymphoid tissue (MALT) and is located in the oropharynx. Because it is exposed to the oral cavity, it is lined by non-keratinized stratified squamous epithelium, which provides protection against mechanical stress and potential microbial invasion. This makes it distinct among lymphoid organs in terms of its epithelial covering.

This epithelial layer also dips down into the tonsil to form crypts, which trap antigens and help initiate immune responses.

❌ Why the Other Options Are Incorrect:

• Thymus:
  Though it contains epithelial reticular cells, it is not lined by stratified squamous epithelium. It is enclosed by a thin connective tissue capsule, not a surface epithelium.

• Lymph nodes:
  These are entirely enclosed in connective tissue capsules and do not have an epithelial lining at all, since they are deep structures not exposed to external surfaces.

• Spleen:
  Like lymph nodes, the spleen has a fibrous capsule, not an epithelial lining. It functions in filtering blood and mounting immune responses but is not exposed to external environments.

• None of these:
  Incorrect because the palatine tonsil clearly fits the description.

The periarteriolar lymphoid sheaths (PALS) are a key component of the white pulp of the spleen. They surround central arterioles and are primarily composed of T lymphocytes, not neutrophils. This region plays a central role in adaptive immunity, especially cell-mediated immune responses.

Thus, the statement that “neutrophils are the predominant cells of PALS” is incorrect.

❌ Why the Other Options Are Incorrect:

• White pulp contains leukocytes: ✅ Correct. It includes lymphocytes, especially T cells (in PALS) and B cells (in follicles), as well as antigen-presenting cells.

• Littoral cells are found in splenic sinusoids: ✅ Correct. These specialized endothelial cells line the splenic sinusoids and help filter aged or damaged red blood cells.

• Red pulp contains sinusoids: ✅ Correct. The red pulp is composed of vascular sinusoids and cords of Billroth, and it is the main site for filtering blood and removing old erythrocytes.

• The spleen contains cords of Billroth: ✅ Correct. These are reticular connective tissue structures in the red pulp that house macrophages, plasma cells, and blood cells.

The lymph node is the only lymphatic organ that has a subcapsular sinus—a space located just beneath the capsule of the node. This sinus receives lymph from afferent lymphatic vessels, allowing it to percolate through the node where it encounters immune cells such as macrophages, dendritic cells, and lymphocytes.

❌ Why the Other Options Are Incorrect:

• Spleen: Filters blood, not lymph. It has red and white pulp, but no subcapsular sinus. The spleen removes old red blood cells and responds to blood-borne antigens.

• Tonsil: These are part of mucosa-associated lymphoid tissue (MALT) and are involved in sampling antigens from the oral and nasal cavities. They do not have afferent lymphatics or a subcapsular sinus.

• Thymus: Involved in T-cell maturation, the thymus lacks lymphatic nodules and does not filter lymph, so it does not have a subcapsular sinus.

• Peyer’s patch: Found in the ileum, Peyer’s patches are part of gut-associated lymphoid tissue (GALT). They contain lymphoid follicles but are not encapsulated like lymph nodes and have no subcapsular sinus.

The spleen is divided into two main components:

• White pulp:
  Contains lymphoid tissue involved in immune functions. The key structures here include:

  • PALS (Periarteriolar lymphoid sheaths): T-cell rich zones surrounding the central arteriole.

  • Lymphocytes are predominantly found here.

• Red pulp:
  Functions mainly in filtering blood, removing old or damaged red blood cells, and storing blood components.

What is Present in Red Pulp?

• Cords of Bilroth (splenic cords):
  These are networks of reticular connective tissue containing macrophages, red blood cells, and other blood cells.

• The red pulp also contains sinusoids (specialized blood vessels) and macrophages that phagocytose damaged erythrocytes.

Why Other Options Are Incorrect:

• Lymphocytes: Mostly found in white pulp, not red pulp.

• Central arteriole: A small artery located within white pulp, not red pulp.

• PALS: Part of white pulp surrounding central arteriole.

• None of these: Incorrect since cords of Bilroth are present in red pulp.

To understand the correct answer, we need to walk through the stages of red blood cell (RBC) development, known as erythropoiesis. This process occurs primarily in the bone marrow and involves several key transitional stages:

1. Proerythroblast – A large cell with a large nucleus and basophilic cytoplasm due to ribosomal RNA.

2. Basophilic erythroblast – Still has a prominent nucleus; cytoplasm remains basophilic.

3. Polychromatophilic erythroblast – Hemoglobin synthesis begins, leading to a mixed cytoplasmic coloration (basophilic + eosinophilic = “polychromatic”).

4. Orthochromatic erythroblast (or normoblast) – Hemoglobin accumulates, cytoplasm becomes more eosinophilic, and the nucleus becomes highly condensed.

5. Reticulocyte – The nucleus is extruded, endoplasmic reticulum is mostly reabsorbed, but the cell retains some residual organelles like ribosomal RNA. It contains approximately 34% hemoglobin, just like a mature erythrocyte, and completes maturation in the bloodstream.

6. Erythrocyte (mature RBC) – Anucleate, biconcave cell with no organelles, fully packed with hemoglobin.

Now let’s relate this to the question.

“A cell is formed having 34% of hemoglobin content, a condensed nucleus, with its endoplasmic reticulum reabsorbed.”

This description aligns most closely with a reticulocyte, but there’s a point of confusion: reticulocytes do not have a nucleus. However, the phrase “a condensed nucleus” may indicate the stage just before nuclear extrusion—yet by the time hemoglobin reaches 34%, the nucleus is already expelled.

Hence, interpreting this question carefully:

• The presence of 34% hemoglobin strongly suggests final maturation.

• Reabsorption of endoplasmic reticulum is typical of reticulocytes.

• The mention of a nucleus may be a distractor or imprecise, potentially meant to convey recent nuclear extrusion.

Therefore, reticulocyte is the best answer.

Why the Other Options Are Incorrect:

• Basophilic erythroblast:
  This stage has no significant hemoglobin. The cytoplasm is deeply basophilic due to abundant ribosomal RNA, and there’s no indication of hemoglobin at 34%. It’s far too early in the developmental sequence.

• Polychromatophilic erythroblast:
  Hemoglobin synthesis has begun but is still incomplete. Cytoplasm has a mixed appearance due to both ribosomes and emerging hemoglobin. Hemoglobin levels are far less than 34%.

• Erythrocyte:
  This mature RBC has no nucleus or organelles, and hemoglobin content is around 34%. However, the question refers to residual ER (suggesting ongoing maturation), which erythrocytes lack. Thus, too far along.

• None of these:
  This is incorrect because the features given do match a known stage of erythropoiesis — the reticulocyte. So one of the choices is clearly valid.

Unlike primary lymphoid organs (like the thymus) or secondary ones (like lymph nodes and spleen), Peyer’s patches are not surrounded by a connective tissue capsule. This makes sense functionally, as they need to interact freely with antigens passing through the intestinal lumen, particularly via specialized M cells that sample antigens.

❌ Why the Other Options Are Incorrect:

• Palatine tonsils
  ➤ These are part of the Waldeyer’s ring and do have a partial capsule on their deep surface (derived from connective tissue), helping separate them from surrounding tissue.

• Spleen
  ➤ Surrounded by a thick connective tissue capsule with trabeculae extending inward. It’s a well-encapsulated lymphoid organ.

• Thymus
  ➤ A primary lymphoid organ with a distinct capsule that sends septa inward to divide it into lobules.

• Lymph node
  ➤ Has a complete capsule made of dense connective tissue, with internal trabeculae supporting its structure.

The thymus is unique among lymphoid organs because it contains a specialized epithelium called the thymic epithelioreticular cells. These cells play essential roles in the maturation of T lymphocytes through both positive and negative selection.

❌ Why the Other Options Are Incorrect:

• Tonsils: Lined by stratified squamous or pseudostratified epithelium, but not specialized in the same sense as the thymus epithelium involved in T-cell maturation.

• Spleen: Does not have a specialized epithelium; consists mainly of red and white pulp with reticular connective tissue.

• Lymph nodes: Structured with a capsule, cortex, and medulla with lymphoid follicles, but no specialized epithelial cells like in the thymus.

• Lymphoid tissue: General term that may include mucosa-associated lymphoid tissue (MALT), which lacks a specialized epithelium like the thymus.

Basophils are a type of granulocytic white blood cell that are identifiable under the microscope by the following features:

• Lobed nucleus (often bilobed, but obscured by granules)
• Large, coarse granules that stain bluish-purple with basic dyes (hence the name baso-phil)
• These granules contain histamine, heparin, and other inflammatory mediators.
• Basophils are involved in type I hypersensitivity reactions (such as anaphylaxis) and participate in immune defense against parasites.

❌ Why the Other Options Are Incorrect:

• Lymphocytes: Round nucleus, scant cytoplasm, no granules, involved in adaptive immunity.

• Neutrophils: Multi-lobed nucleus, pale cytoplasmic granules, key players in bacterial defense—not histamine release.

• Eosinophils: Bilobed nucleus, red-orange granules, combat parasites, and participate in allergic responses—but not primarily through histamine.

• Monocytes: Largest WBCs, kidney-shaped nucleus, no prominent granules, differentiate into macrophages in tissues.

Hematopoiesis is the process by which all formed (solid) elements of the blood are produced. This includes:

• Erythrocytes (red blood cells)
• Leukocytes (white blood cells)
• Thrombocytes (platelets)

This process occurs primarily in the bone marrow in adults and begins from multipotent hematopoietic stem cells that differentiate into various blood cell lineages.

Breakdown of the Term:

• “Hema” = blood
• “Poiesis” = formation

Why the Other Options Are Incorrect:

• Erythropoiesis: Refers only to red blood cell formation, not all blood cells.
• Homeostasis: Refers to the body’s maintenance of internal stability, not blood cell production.
• Hemostasis: Refers to the cessation of bleeding, involving clot formation — not related to blood cell production.
• Leukopoiesis: Refers specifically to the formation of white blood cells.

While erythropoiesis and leukopoiesis are parts of hematopoiesis, the comprehensive term for formation of all blood elements is hematopoiesis.

The biconcave, disc-like cells observed under the microscope are characteristic of erythrocytes, also known as red blood cells (RBCs).

Key Features of Erythrocytes:

• They are anucleate (no nucleus) in mammals.
• Their biconcave shape increases the surface area for gas exchange and allows them to deform as they pass through capillaries.
• They are stained pink with eosin due to their high hemoglobin content.

This unique shape is highly efficient for their primary function: the transport of oxygen (O₂) and carbon dioxide (CO₂) throughout the body.

Why the Other Options Are Incorrect:

• Lymphocytes: These are round white blood cells with a large nucleus and scant cytoplasm—not biconcave.
• Megakaryocytes: These are very large cells found in the bone marrow, responsible for producing platelets. They are not found in peripheral blood and are not disc-shaped.
• Monocytes: These are large white blood cells with a kidney-shaped nucleus, not biconcave.
• Leukocytes: A general term for all white blood cells, which includes lymphocytes, monocytes, neutrophils, etc., but none are biconcave.

🔹 Why Thymus is correct:

The thymus is a primary lymphoid organ responsible for T-lymphocyte maturation. Histologically, it is unique among lymphoid organs due to the following features:

• It is lobulated, with each lobule showing:

  • An outer cortex: Dark-staining due to densely packed immature T cells (thymocytes).

  • An inner medulla: Lighter-staining because it has fewer lymphocytes and more epithelial cells.

• Concentric acidophilic bodies seen in the medulla are called Hassall’s corpuscles. These are whorls of keratinized epithelial reticular cells, and their function is not fully understood but may be related to T-cell tolerance.

These features are hallmarks of the thymus, especially in pediatric histology.

❌ Why the other options are incorrect:

• Spleen
  The spleen has white pulp (lymphoid follicles) and red pulp (vascular sinusoids), not lobules with cortical-medullary architecture or Hassall’s corpuscles.

• Tonsils
  Contain crypts and lymphoid follicles, but no lobules or Hassall’s corpuscles. They are covered by stratified squamous or respiratory epithelium, depending on the type.

• Bone marrow
  Site of hematopoiesis, with hematopoietic cords and sinusoids, not lymphoid lobules or Hassall’s corpuscles.

• Lymph nodes
  Have cortex, paracortex, and medulla, but are not organized into lobules, and lack Hassall’s corpuscles.

🔹 Why Pharyngeal tonsil is correct:

The pharyngeal tonsil, located in the nasopharynx, is uniquely lined by pseudostratified columnar epithelium with cilia, also known as respiratory epithelium. This is because of its anatomical position in the upper respiratory tract, which requires an epithelium specialized for mucociliary clearance to trap and remove airborne particles and pathogens.

This histological feature helps distinguish the pharyngeal tonsil from the other tonsils, which are covered by different types of epithelium due to their locations and functions.

❌ Why the other options are incorrect:

• Lymph nodes
  Lymph nodes are internal organs and do not have an epithelial covering like tonsils. Instead, they are surrounded by a fibrous capsule and internally composed of lymphoid tissue with no surface epithelium.

• Lingual tonsil
  Located at the posterior part of the tongue, it is covered by stratified squamous non-keratinized epithelium, appropriate for protection in the oral cavity.

• Spleen
  Like lymph nodes, the spleen is an internal organ with no surface epithelium. It has a connective tissue capsule and is involved in filtering blood, not directly exposed to the external environment.

• Palatine tonsil
  Found between the palatoglossal and palatopharyngeal arches, the palatine tonsil is covered by stratified squamous non-keratinized epithelium, designed to resist friction and trauma from food particles.

In histology, we often use a staining technique called Hematoxylin and Eosin (H&E) staining:

• Hematoxylin is a basic dye. It binds to acidic (basophilic) structures.
• These structures stain blue or purplish-blue.

So when we say a structure is basophilic, we mean it has an affinity for basic dyes like hematoxylin—and will appear blue under the microscope.

❌ Why the Other Options Are Incorrect:

1. Deep purple

• Not a standard H&E staining result.
• May be seen in other special stains, but not typically how basophilia is described.

2. Pink

• This is the color of acidophilic (eosinophilic) structures, which bind eosin, the acidic dye.
• Cytoplasm, collagen, and muscle fibers often stain pink.

3. Lilac

• Not a typical H&E term.
• Sometimes used descriptively, but not a specific indicator of basophilia.

4. Red

• Another eosinophilic color—often associated with erythrocytes or muscle fibers stained by eosin, not hematoxylin.

The key concept in this question is central tolerance, which occurs in the thymus for T cells and in the bone marrow for B cells.

1. Central tolerance ensures that B and T cells that have receptors capable of binding to self-antigens (molecules found in the body) are eliminated early in development.

2. If a T or B cell recognizes self-antigens too strongly, it can undergo negative selection, which results in apoptosis (programmed cell death) or differentiation into regulatory cells that help suppress immune responses against self.

❌ Why the Other Options Are Incorrect:

1. Induced tolerance

• This term refers to tolerance that is actively induced through external signals or therapies (e.g., in organ transplantation or in cases of immunosuppression). It is not a process occurring naturally during lymphocyte development.

2. Peripheral tolerance

• This is the process where self-reactive T or B cells that escape central tolerance (in the thymus or bone marrow) can be controlled in the periphery (outside the central organs, i.e., tissues and circulation) through mechanisms like anergy, deletion, or regulatory T cells. It happens after the cells have matured.

3. Autoimmunity

• Autoimmunity refers to the dysregulation of immune tolerance, where the immune system mistakenly attacks self-antigens. It is the consequence of failure of tolerance, not the process that eliminates self-reactive cells.

4. Self-tolerance

• While self-tolerance is the overall concept of the immune system’s ability to ignore self-antigens, the specific process that eliminates self-reactive lymphocytes during their development is central tolerance. “Self-tolerance” is more of a broad concept encompassing both central and peripheral tolerance.

Red blood cells (erythrocytes) are specialized cells designed exclusively for oxygen transport. To maximize their efficiency, they exhibit several distinct structural adaptations, and one of the most important is the absence of a nucleus in mature form.

No nucleus (Correct Answer)

Mature RBCs in humans lack a nucleus. This adaptation provides more space for hemoglobin, the oxygen-carrying molecule. It also allows the cells to assume their distinctive shape and maximize gas exchange efficiency. Without a nucleus, RBCs cannot divide or repair themselves, which is why they have a finite lifespan (around 120 days).

Why the Other Options Are Incorrect:

Biconvex shape

Incorrect — RBCs have a biconcave shape, not biconvex. This shape increases the surface area-to-volume ratio, enhancing oxygen and carbon dioxide diffusion. The biconcave form also contributes to flexibility and deformability as RBCs pass through narrow capillaries.

No flexibility

Incorrect — Flexibility is actually a key feature of RBCs. Their cytoskeleton (especially the spectrin network) enables them to deform as they squeeze through narrow blood vessels. Without flexibility, they would get trapped in capillaries and not complete circulation efficiently.

Myoglobin present

Incorrect — Myoglobin is found in muscle cells, not RBCs. It serves a similar function to hemoglobin (oxygen storage), but it is a monomeric protein with a different role. RBCs rely solely on hemoglobin to transport oxygen.

Mitochondria present

Incorrect — Mature RBCs lack mitochondria. This forces them to generate energy through anaerobic glycolysis, which is an advantage because it ensures they don’t consume the oxygen they are meant to deliver.

In histology, different stains are used to visualize cell components under a microscope. Some stains are unique or more selective for specific structures.

Azurophilia is a term used for structures (like lysosomes in white blood cells) that take up a purple stain, especially with Romanowsky-type stains such as Wright’s or Giemsa stain. These granules are found in neutrophils, monocytes, and lymphocytes, and represent primary lysosomes.

This is a unique stain characteristic, making it helpful in differentiating between types of white blood cells and understanding intracellular organelles.

❌ Why the Other Options Are Incorrect:

• Neutrophilia (pink stain) for neutrophil granules
  ✘ “Neutrophilia” is a clinical term referring to increased neutrophils in blood, not a stain. The granules in neutrophils stain light lilac or neutral pink due to the balanced affinity for acidic and basic dyes, not because of a specific stain called “neutrophilia.”

• Hemophilia (red stain) for hemoglobin
  ✘ “Hemophilia” is a genetic bleeding disorder, not a stain. Hemoglobin does stain pink-red with eosin, but there’s no stain called “hemophilia.”

• Eosinophilia (yellow stain) for hemoglobin
  ✘ Eosinophilia refers to affinity for eosin (an acidic dye), which stains cytoplasmic components pink, especially in eosinophils. The color is pink or red, not yellow, and hemoglobin itself is eosinophilic, not uniquely stained yellow.

• Basophilia (blue stain) for DNA in ribosomes
  ✘ Basophilia refers to structures that attract basic dyes like hematoxylin, which stain them blue. While it’s true that DNA and RNA are basophilic, ribosomes contain RNA, not DNA. Also, basophilia isn’t “unique” to ribosomes — it’s common to nuclei and rough ER.

The cell described — anucleated, biconcave, disc-shaped — is characteristic of an erythrocyte, commonly known as a red blood cell (RBC).

Key features of erythrocytes:

• Anucleated: Mature RBCs lack a nucleus, which increases space for hemoglobin and improves flexibility.

• Biconcave shape: Maximizes surface area-to-volume ratio, facilitating efficient gas exchange (O₂ and CO₂).

• Disc-shaped: Enhances the ability to deform as they pass through narrow capillaries.

These adaptations allow erythrocytes to function as efficient oxygen transporters in the circulatory system.

❌ Why the Other Options Are Incorrect:

• Neutrophil
  ✘ A type of white blood cell with a multi-lobed nucleus and granular cytoplasm. It is nucleated, not anucleated.

• Macrophage
  ✘ Large nucleated phagocytic cells found in tissues. They are not present in blood smears in their mature form.

• Thrombocyte (Platelet)
  ✘ While anucleated like RBCs, platelets are small cell fragments, not disc-shaped cells, and lack the biconcave structure.

• Leukocyte
  ✘ A general term for white blood cells, all of which are nucleated and do not have the biconcave shape.

The organ described — with an outer darkly staining cortex and an inner lightly staining medulla, featuring acidophilic structures (Hassall’s corpuscles) — is the thymus.

This structure is unique among lymphoid organs in both its histology and function.

🔬 Histological Features of the Thymus:

• Cortex (outer, dark-staining):

  • Densely packed with immature T lymphocytes (thymocytes), making it basophilic (dark).

  • Contains epithelial reticular cells and macrophages.

• Medulla (inner, light-staining):

  • Contains fewer lymphocytes, hence stains more lightly.

  • Unique feature: Hassall’s corpuscles, which are acidophilic, whorled structures composed of degenerated epithelial cells.

❌ Why the Other Options Are Incorrect:

• Bone marrow
  ✘ Site of hematopoiesis, lacks the sharply demarcated cortex-medulla structure and acidophilic Hassall’s corpuscles.

• Spleen
  ✘ Contains white pulp and red pulp, but not organized into cortex and medulla.

• Liver
  ✘ Not a lymphoid organ and lacks any cortex-medulla organization.

• Lymph node
  ✘ Does have a cortex and medulla, but no Hassall’s corpuscles and does not display the same acidophilic patches found in the thymus.

All blood cells — including red blood cells (RBCs), white blood cells (WBCs), and platelets — originate from a single type of cell found primarily in the bone marrow known as the pluripotent hematopoietic stem cell (PHSC).

These stem cells have two key properties:

1. Self-renewal – They can divide to produce more stem cells.

2. Multipotency – They can differentiate into all the various blood cell lineages.

🧬 Differentiation Pathway:

1. Pluripotent hematopoietic stem cell
   ↓

2. Multipotent progenitor cells
   ↓

3. Committed progenitor cells (like CFUs)
   ↓

4. Mature blood cells (e.g., neutrophils, lymphocytes, erythrocytes, platelets)

❌ Why the Other Options Are Incorrect:

• Colony forming unit (CFU)
  ✘ These are progenitor cells, a few steps downstream from stem cells. They are already committed to specific lineages like CFU-GEMM (granulocyte, erythrocyte, monocyte, megakaryocyte). They do not give rise to all blood cells.

• Committed stem cells
  ✘ These are differentiated forms of hematopoietic stem cells that are already committed to either the myeloid or lymphoid lineage. They don’t retain the ability to form all blood cells.

• Growth and differentiation inducer cells
  ✘ These are signaling molecules or cytokines (like interleukins, erythropoietin) that regulate stem cells — but they are not the source of blood cells themselves.

• Megakaryocytes
  ✘ These are large bone marrow cells that produce platelets — but they are terminally differentiated and do not give rise to all blood cells.

• The term “polymorph“ is short for polymorphonuclear leukocyte, which specifically refers to neutrophils.

• Neutrophils have a multi-lobed nucleus (usually 2–5 lobes), hence the name.

• Eosinophils, while still granulocytes, typically have a bi-lobed nucleus, not polymorphic in the same sense.

Therefore, this statement is incorrect.

✅ Why the Other Statements Are Correct:

✅ “They contain non-specific azurophilic granules”

• True. All granulocytes contain azurophilic (primary) granules, which are lysosome-like and non-specific.

• These granules contain enzymes such as myeloperoxidase, lysozyme, and defensins.

✅ “They contain a single nucleus with multiple lobes”

• True. All granulocytes (neutrophils, eosinophils, basophils) are mononuclear (one nucleus) but lobulated.

• Neutrophils: 3–5 lobes, Eosinophils: usually bilobed, Basophils: irregular/lobed.

✅ “Granulocytes are also called myeloid cells”

• True. Granulocytes develop from the myeloid lineage in hematopoiesis.

• Myeloid cells include granulocytes, monocytes, erythrocytes, megakaryocytes, etc.

✅ “They contain specific granules”

• True. Each type of granulocyte has distinctive, specific granules:

  • Neutrophils: small, pale granules with antimicrobial enzymes.

  • Eosinophils: large, red-orange granules with major basic protein.

  • Basophils: large, blue-black granules with histamine and heparin.

IgA (Immunoglobulin A) is the primary antibody found in external secretions, such as:

• Tears

• Saliva

• Mucus

• Breast milk

• Gastrointestinal and respiratory tract secretions

This makes it essential for mucosal immunity, where it plays a key role in neutralizing pathogens before they invade epithelial surfaces.

❌ Why the Other Options Are Incorrect:

❌ IgG

• The most abundant antibody in serum and tissue fluids.

• Important in systemic immunity but not prominent in secretions like saliva or tears.

❌ IgD

• Primarily found on naïve B cells as a receptor.

• Has minimal presence in secretions and a still poorly understood role.

❌ IgM

• The first antibody produced in response to infection.

• Exists mostly in plasma as a pentamer.

• Too large to be effectively secreted into mucosal fluids.

❌ IgE

• Involved in allergic reactions and parasitic infections.

• Binds to mast cells and basophils, not typically found in mucosal secretions like tears or saliva.

Platelets, also called thrombocytes, are small, anucleate cell fragments that play a critical role in hemostasis (blood clotting). They are derived from megakaryocytes, which are large cells found in the bone marrow.

Megakaryocyte to Platelet Formation:

• Megakaryocytes extend long projections called proplatelets into the bloodstream within bone marrow sinusoids.
• These proplatelets fragment into thousands of platelets.
• This process is regulated by thrombopoietin, a hormone produced mainly by the liver.

Why the Other Options Are Incorrect:

• Reticulocytes:
  • These are immature red blood cells, not involved in platelet formation.
• Monocytes:
  • These are a type of white blood cell that differentiate into macrophages or dendritic cells in tissues.
• Macrophages:
  • These are mature mononuclear phagocytes, involved in immune defense, not thrombopoiesis.
• Myeloid cells:
  • This is a broad category that includes precursors of granulocytes, monocytes, erythrocytes, and megakaryocytes, but platelets are not directly formed from “myeloid cells”—they specifically arise from megakaryocytes.

Granulocytes are a subgroup of white blood cells that include neutrophils, eosinophils, and basophils. They are called “granulocytes” because they have granules in their cytoplasm that are visible under a microscope after staining.

Correct Statements:

• They contain non-specific azurophilic granules:
  • True. These are primary granules found in all granulocytes and contain enzymes like myeloperoxidase and lysozyme.
• They contain specific granules:
  • True. These are secondary granules unique to each granulocyte type (e.g., lactoferrin in neutrophils, major basic protein in eosinophils).
• Granulocytes are also called myeloid cells:
  • True. Granulocytes are derived from myeloid lineage in the bone marrow.
• They contain a single nucleus with multiple lobes:
  • True. Granulocytes like neutrophils are polymorphonuclear leukocytes, having a lobulated single nucleus.

Incorrect Statement:

• The term polymorph refers to eosinophils:
  • False. The term “polymorph” or “polymorphonuclear leukocyte” (PMN) specifically refers to neutrophils, due to their multi-lobed nucleus.
  • Eosinophils, while bilobed, are not typically referred to as “polymorphs.”

Reticulocytes are immature red blood cells that are released from the bone marrow into the bloodstream. They represent a key transitional stage between nucleated erythroid precursors and mature erythrocytes.

What’s True About Reticulocytes:

• Immature form of erythrocytes:
  • Correct. Reticulocytes are enucleated but still contain remnants of RNA and organelles involved in protein synthesis.
• Stained with supravital staining:
  • Correct. Supravital stains like new methylene blue are used to visualize the residual RNA network, giving a reticulated appearance.
• Constitute <1% of all blood cells:
  • Correct. Reticulocytes typically make up about 0.5–2% of red blood cells in peripheral blood in healthy individuals.
• Contain organelles for hemoglobin synthesis:
  • Correct. Reticulocytes retain ribosomes, mitochondria, and some Golgi apparatus, which are necessary for the final stages of hemoglobin synthesis.
• What’s Incorrect:

• They synthesize 50% of red blood cell’s hemoglobin:
  • Incorrect. The majority of hemoglobin synthesis occurs in earlier erythroid precursors in the bone marrow. Reticulocytes contribute only a small fraction (~10–20%) of total hemoglobin production. By the time they enter circulation, most hemoglobin synthesis is already complete.

The cell cycle is a tightly controlled sequence of events that leads to cell division. It consists of:

• G1 phase (Gap 1): Cell grows and prepares for DNA replication
• S phase (Synthesis): DNA replication occurs
• G2 phase (Gap 2): Cell prepares for mitosis, checking for DNA damage
• M phase (Mitosis): Cell divides into two daughter cells

Progression through these phases is governed by:

• Cyclins: Regulatory proteins that fluctuate in concentration through the cycle
• Cyclin-dependent kinases (CDKs): Enzymes that must bind cyclins to become active
• CDK inhibitors (CKIs): Proteins that suppress CDK activity to prevent premature progression

This system ensures the cell only divides when conditions are right, maintaining genomic integrity.

Why the Other Options Are Incorrect:

• In S phase, there is synthesis of organelles: Organelles are primarily synthesized during G1 and G2, not S phase. S phase is for DNA replication.
• In mitosis, the entire cell is divided into four daughter cells: Mitosis produces two daughter cells. Four cells result from meiosis, not mitosis.
• G1 is the premitotic phase: G1 is before DNA synthesis (pre-S phase), not pre-mitotic. G2 is the true premitotic phase.
• G2 is the growth phase: While some growth occurs, G1 is the major growth phase. G2 is more about checking DNA integrity and preparing for mitosis.

Eosinophils are white blood cells that play a crucial role in defense against parasites and in allergic reactions. They contain large, membrane-bound cytoplasmic granules, which house several toxic proteins:

• Major Basic Protein (MBP)
• Eosinophil Cationic Protein (ECP)
• Eosinophil-Derived Neurotoxin (EDN)
• Eosinophil Peroxidase (EPO)

Among these, Eosinophil Cationic Protein (ECP) is a major cationic protein with cytotoxic, antiviral, and ribonuclease activity. It can damage parasite membranes and is involved in the tissue damage seen in allergic conditions like asthma.

Why the Other Options Are Incorrect:

• Monocytes are phagocytic cells that mature into macrophages but do not have granules rich in major cationic proteins.
• Neutrophils have azurophilic and specific granules containing enzymes like myeloperoxidase and elastase, but not major cationic protein.
• Basophils release histamine and leukotrienes during allergic reactions, but their granules do not contain major cationic protein.
• Mast cells are similar to basophils but found in tissues; they also release histamine and cytokines — again, not major cationic protein.

To answer this, we need to understand the concept of lymphoid nodules (or follicles). These are organized collections of B-lymphocytes typically found in secondary lymphoid organs where immune responses are generated. They often have germinal centers, where B cells proliferate, differentiate, and undergo affinity maturation.

Let’s evaluate each structure:

Tonsils

• Nodulated
• Contain lymphoid follicles with germinal centers.
• Part of the MALT (mucosa-associated lymphoid tissue).
• Involved in initiating immune responses to antigens entering through the mouth or nose.

Spleen

• Nodulated
• Contains white pulp, where you’ll find lymphoid follicles (nodules) centered around central arterioles.
• Important in filtering blood and mounting immune responses to blood-borne pathogens.

Lymph node

• Nodulated
• The cortex contains many primary and secondary follicles.
• Secondary follicles have germinal centers active during immune responses.

ThymusNot nodulated

(Correct Answer)

• The thymus is a primary lymphoid organ, essential for T-cell maturation.
• It does not contain lymphoid follicles or germinal centers, because it is not involved in B-cell activation.
• The thymus is organized into a cortex and medulla, with Hassall’s corpuscles in the medulla — a distinctive feature.

“None of them”

• Incorrect. Since the thymus is clearly not nodulated, this option cannot be correct.

Eosinophils are granulocytes involved primarily in parasitic defense and allergic responses. Their granules are rich in cytotoxic proteins, one of which is Major Cationic Protein (also known as Major Basic Protein).

❌ Why the Other Options Are Incorrect:

❌ Neutrophils

• Granules contain myeloperoxidase, elastase, lysozyme, and other enzymes.

• Important in bacterial defense, but do not contain major cationic protein.

❌ Basophils

• Contain histamine, heparin, leukotrienes, and other vasoactive substances.

• Involved in hypersensitivity reactions, not in parasitic cytotoxicity.

• No MCP.

❌ Mast cells

• Similar to basophils in function and content — histamine, tryptase, prostaglandins.

• Crucial in IgE-mediated allergic responses, not MCP carriers.

❌ Monocytes

• Agranular cells that differentiate into macrophages.

• Involved in phagocytosis and antigen presentation, not granule-mediated cytotoxicity.

• No cationic protein-containing granules.

The thymus is unique among lymphoid organs in that it contains a specialized epithelial framework, rather than a typical reticular connective tissue stroma found in other lymphoid tissues. The specialized epithelium plays a functional role in T cell education.

❌ Why the Other Options Are Incorrect:

❌ Lymphoid tissue

• General term that can describe tissues containing lymphocytes — such as MALT, BALT, GALT — not a specific organ.

• Typically supported by reticular connective tissue, not epithelial tissue.

❌ Tonsils

• Have surface epithelium (e.g., stratified squamous in palatine tonsils) but do not have an internal epithelial framework.

• Lymphoid follicles and crypts are embedded in reticular connective tissue, not specialized epithelial scaffolds.

❌ Spleen

• Has white pulp and red pulp supported by a reticular connective tissue framework.

• Functions in blood filtration, not T-cell maturation, and contains no epithelial stroma.

❌ Lymph nodes

• Have a cortex and medulla with lymphoid follicles and sinuses.

• Framework made of reticular fibers, produced by fibroblasts.

• No epithelial cells forming the structural framework.

Major Basic Protein (MBP) is a hallmark of eosinophils, not basophils.

✅ Why the Other Options Are Correct:

• Releases histamine ✅
  Basophils degranulate upon IgE-mediated activation, releasing histamine, promoting inflammation and allergic reactions.

• Has basophilic cytoplasm ✅
  Yes — “basophilic” means the cytoplasm stains with basic dyes, which is characteristic.

• Contains blue/purple staining granules ✅
  The granules are rich in acidic contents, thus bind basic dyes and appear deep blue/purple.

• Contains lobed nucleus ✅
  True — although often obscured by granules, the nucleus is typically bilobed or irregular.

Morphological Characteristics:

• Lobed nucleus: Typically bilobed, but often obscured by granules.
• Large, purple-blue cytoplasmic granules: These granules are basophilic (stain with basic dyes) and can sometimes make the nucleus hard to see.

Functional Role:

• Basophils release histamine, heparin, and other mediators during allergic and inflammatory reactions.
• They participate in Type I hypersensitivity reactions (like anaphylaxis) and are involved in IgE-mediated responses.

Why the Other Options Are Incorrect:

Monocytes:

Large, kidney-shaped or indented nucleusNo granules like basophilsFunction: Differentiate into macrophages, phagocytose pathogens and debris.

Eosinophils:

Bilobed nucleus

Red-orange granules (eosinophilic staining)

Involved in parasitic infections and allergic conditions, but do not primarily release histamine.

Lymphocytes:Large, round nucleus with a thin rim of cytoplasmNo granules Function in adaptive immunity (B and T cells).Neutrophils:Multilobed nucleus (3–5 lobes Fine, pale granules (not purple-blue)Function as first responders to bacterial infection; they do not release histamine.

Clinical Relevance:

Elevated basophil counts are seen in:

• Chronic myelogenous leukemia (CML)
• Allergic reactions
• Parasitic infections (rarely)

Platelets are very small, measuring about 2–3 micrometers in diameter, making them much smaller than most other blood cells. Their small size is crucial for their function in navigating through capillaries and contributing to clot formation.

• Platelets do not have a nucleus, so Option A is incorrect and thus the right choice here.
• All other options accurately describe features of platelets.Why other options are wrong

 2–3 µm in greatest diameter — Correct

Yes, this is accurate. Platelets are very small, measuring about 2–3 micrometers in diameter, making them much smaller than most other blood cells. Their small size is crucial for their function in navigating through capillaries and contributing to clot formation.

Biconvex discs — Correct

This is a common point of confusion. While red blood cells (RBCs) are famously biconcave, platelets are described as biconvex or discoid when inactive. Upon activation (during clotting), they become irregular and sticky. So, this option is also correct.

 Formed from megakaryocytes — Correct

Absolutely right. Platelets are cytoplasmic fragments that bud off from the large bone marrow cells called megakaryocytes. These megakaryocytes undergo a process called endomitosis, resulting in a large cell with multiple copies of DNA and abundant cytoplasm, which fragments to form platelets.

Not true cells — Correct

This is also correct. Since platelets lack a nucleus and most other organelles, they are considered cell fragments rather than true, fully functioning cells. They are essentially functional cytoplasmic fragments equipped with granules and surface receptors, not full eukaryotic cells.

• Pharyngeal tonsil is covered by pseudostratified ciliated columnar epithelium (respiratory epithelium).

• This makes sense functionally, since it’s exposed to inspired air and needs mucociliary clearance.

❌ Spleen — Incorrect

• The spleen is a lymphoid organ, but it is not covered by any epithelium.

• It is surrounded by a connective tissue capsule.

• Internally, it has white pulp and red pulp, but no epithelial covering.

❌ Palatine tonsil — Incorrect

• Covered by non-keratinized stratified squamous epithelium.

• This protects it from mechanical stress and exposure to food and air.

❌ Lymph nodes — Incorrect

• Like the spleen, lymph nodes are encapsulated by dense connective tissue, not epithelium.

• Their outer capsule is non-epithelial, and internally, they have cortex, paracortex, and medulla, but no epithelial lining.

❌ Lingual tonsil — Incorrect

• Covered by stratified squamous epithelium, often non-keratinized, but sometimes partially keratinized depending on location.

• This protects it from friction and trauma associated with mastication and speech.

🔬 What are Monocytes?

• Monocytes are a type of white blood cell (leukocyte), part of the innate immune system.

• They circulate in the bloodstream and have a large, kidney-shaped nucleus.

🔄 What happens when they enter tissue?

• Once monocytes exit the bloodstream and migrate into tissues, they differentiate into macrophages.

• These macrophages:

  • Perform phagocytosis (engulfing pathogens and debris),

  • Present antigens to T-cells,

  • Secrete cytokines to coordinate inflammation and healing.

🧬 Examples of tissue-specific macrophages:

❌ Why the Other Options Are Incorrect:

• Basophils:
   → Involved in allergic reactions; contain histamine and heparin.
   → They do not become macrophages.

• Reticulocytes:
   → These are immature red blood cells, not involved in immunity.
   → They mature into erythrocytes, not macrophages.

• Lymphocytes:
   → Include T-cells, B-cells, and NK cells, involved in adaptive immunity.
   → They do not transform into macrophages.

• Neutrophils:
   → First responders in acute inflammation, perform phagocytosis, but they are terminal cells—they do not differentiate further into macrophages.

🧬 What are CD4 Cells?

• CD4 is a cell surface glycoprotein that acts as a coreceptor for the T-cell receptor (TCR).

• CD4+ cells are a subset of T-lymphocytes, specifically known as helper T cells (Th cells).

• They coordinate the immune response by activating macrophages, B cells, and cytotoxic T cells via cytokine signaling.

🔬 CD Markers:

• CD4+ cells = Helper T cells

• CD8+ cells = Cytotoxic T cells

• B cells typically express CD19, CD20, and CD21, not CD4.

❌ Why the Other Options Are Incorrect:

• Granulocytes:
   → Include neutrophils, eosinophils, and basophils. They are part of the innate immune system and lack specific CD4 markers.

• Erythrocytes:
   → Red blood cells that carry oxygen. They do not express CD markers like CD4.

• Megakaryocytes:
   → Large bone marrow cells that give rise to platelets. No role in adaptive immunity or CD4 expression.

• B-lymphocytes:
   → Adaptive immune cells that produce antibodies. They express different CD markers (like CD19, CD20) but not CD4.