Immunopathology — MCQs

A 24-year-old woman with leukemia receives an allogeneic bone marrow transplant. Three weeks later, she develops a skin rash and diarrhea. Liver function tests show elevated serum levels of AST and ALT. A skin biopsy discloses a sparse lymphocytic infiltrate in the dermis and epidermis, as well as apoptotic cells in the epidermal basal cell layer. Skin rash and diarrhea in this patient are caused primarily by which of the following cells?

Q32Easy

Antibodies in Immune Thrombocytopenic Purpura (ITP) are typically:

Q33Easy

Which of the following is NOT an antigen for antiphospholipid antibodies?

Q34Medium

All of the following are true EXCEPT:

Q35Easy

Which of the following is a flow cytometric B cell marker?

Q36Easy

Changes in which structural level of amyloid protein makes it insoluble?

Q37Easy

In which year was HIV discovered?

Q38Easy

Which of the following statements is NOT true regarding Natural Killer (NK) cells?

Q39Easy

Henoch-Schonlein purpura is characterized by deposition of which immunoglobulin?

Q40Easy

CD8 is a marker for which of the following cell types?

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 31: A 24-year-old woman with leukemia receives an allogeneic bone marrow transplant. Three weeks later, she develops a skin rash and diarrhea. Liver function tests show elevated serum levels of AST and ALT. A skin biopsy discloses a sparse lymphocytic infiltrate in the dermis and epidermis, as well as apoptotic cells in the epidermal basal cell layer. Skin rash and diarrhea in this patient are caused primarily by which of the following cells?

A. Donor lymphocytes (Correct Answer)
B. Donor plasma cells
C. Fixed tissue macrophages
D. Recipient lymphocytes

Explanation: **Explanation:** The clinical presentation of skin rash, diarrhea, and elevated liver enzymes (the "classic triad") following an allogeneic bone marrow transplant (BMT) is diagnostic of **Graft-versus-Host Disease (GVHD)** [1]. **1. Why Donor Lymphocytes is Correct:** GVHD occurs when immunologically competent cells (specifically **Donor T-lymphocytes**) are transplanted into a recipient who is immunocompromised [2]. These donor T-cells recognize the recipient's HLA antigens as foreign and mount an immune attack. * **Acute GVHD** typically occurs within 100 days of BMT [1]. * The primary targets are the **skin** (rash/dermatitis), **GIT** (diarrhea/mucositis), and **liver** (jaundice/elevated enzymes) [1]. * Histologically, the presence of **apoptotic keratinocytes** (Civatte bodies) in the basal layer of the epidermis is a hallmark of T-cell mediated cytotoxicity [1], [2]. **2. Why Incorrect Options are Wrong:** * **Donor Plasma Cells:** These produce antibodies. GVHD is primarily a Type IV (cell-mediated) hypersensitivity reaction driven by T-cells, not a humoral response. * **Fixed Tissue Macrophages:** While they may act as antigen-presenting cells, they are not the primary effectors of the systemic damage seen in GVHD. * **Recipient Lymphocytes:** In a BMT patient, the recipient’s immune system is usually ablated by chemotherapy or radiation. If recipient lymphocytes were active against the donor tissue, it would result in **Graft Rejection**, not GVHD. **Clinical Pearls for NEET-PG:** * **Prerequisites for GVHD (Billingham’s Criteria):** 1. Graft must contain immunocompetent cells. 2. Recipient must be HLA-incompatible. 3. Recipient must be immunocompromised. * **Common Scenarios:** Allogeneic BMT and non-irradiated blood transfusions in immunocompromised patients. * **Chronic GVHD:** Occurs after 100 days; resembles systemic sclerosis (fibrosis) or autoimmune disorders. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 182-183. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Disorders Involving Inflammatory And Haemopoietic Cells, pp. 654-655.

Question 32: Antibodies in Immune Thrombocytopenic Purpura (ITP) are typically:

A. IgG (Correct Answer)
B. IgM
C. IgE
D. IgD

Explanation: **Explanation:** **1. Why IgG is Correct:** Immune Thrombocytopenic Purpura (ITP) is a Type II hypersensitivity reaction characterized by the production of autoantibodies against platelet surface glycoproteins, most commonly **GPIIb/IIIa** or **GPIb/IX**. These antibodies are predominantly of the **IgG** class. Once coated with IgG, platelets are recognized by the Fc̲̲ receptors on splenic macrophages, leading to their premature sequestration and destruction in the spleen [1]. IgG is the only antibody class capable of crossing the placenta, which explains why newborns of mothers with ITP may suffer from transient neonatal thrombocytopenia. **2. Why Other Options are Incorrect:** * **IgM:** While IgM is the first antibody produced in a primary immune response and is involved in cold agglutinin disease, it is not the primary mediator in ITP. * **IgE:** This isotype is associated with Type I hypersensitivity (allergies, asthma) and parasitic infections, not autoimmune platelet destruction. * **IgD:** Found primarily on the surface of B-cells as an antigen receptor; it has no known role in the pathogenesis of ITP. **3. NEET-PG High-Yield Pearls:** * **Site of Destruction:** The **Spleen** is the major site of both antibody production and platelet destruction in ITP [2]. Splenectomy is often effective because it removes both the source of antibodies and the site of clearance [1]. * **Bone Marrow:** Shows **increased megakaryocytes** (compensatory hyperplasia) to counter peripheral destruction [1] [2]. * **Clinical Presentation:** Characterized by mucocutaneous bleeding (petechiae, purpura, epistaxis) with a **normal** PT/aPTT and **low** platelet count [1] [2]. * **Treatment:** First-line therapy includes Corticosteroids or IVIG (which blocks the Fc receptors on macrophages) [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 665-667. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 619-621.

Question 33: Which of the following is NOT an antigen for antiphospholipid antibodies?

A. Prothrombin
B. Erythrocyte membrane (Correct Answer)
C. Phospholipid
D. Beta 2 glycoprotein

Explanation: ### Explanation **Antiphospholipid Syndrome (APS)** is an autoimmune hypercoagulable state characterized by the presence of antiphospholipid antibodies (aPL). Despite the name, these antibodies are primarily directed against **plasma proteins** that have an affinity for anionic phospholipids, rather than the phospholipids alone. **Why Erythrocyte Membrane is the Correct Answer:** Antiphospholipid antibodies do not target the erythrocyte membrane. Antibodies directed against the erythrocyte membrane are typically seen in **Autoimmune Hemolytic Anemia (AIHA)** (e.g., anti-Rh or anti-I antibodies) [2]. While APS can coexist with other autoimmune conditions like SLE, the erythrocyte membrane itself is not a diagnostic antigen for aPL [2]. **Analysis of Incorrect Options:** * **Phospholipid:** Although the name suggests otherwise, "pure" phospholipids are rarely the sole target. However, in laboratory assays (like the VDRL/RPR for syphilis), **cardiolipin** (a phospholipid) is used as the antigen, making it a classic target in the context of "false-positive" syphilis tests [1]. * **Beta 2 Glycoprotein I ($\beta_2$-GPI):** This is the **most important** and most common protein cofactor targeted by aPL. It is a plasma protein that binds to anionic surfaces; its binding to phospholipids exposes the epitopes that these antibodies recognize. * **Prothrombin:** This is another significant protein cofactor. Antibodies against prothrombin (anti-phosphatidylserine-prothrombin complex) are frequently found in APS patients and contribute to the prothrombotic state. **High-Yield Clinical Pearls for NEET-PG:** * **The Paradox:** In vitro, aPL interferes with phospholipids to **prolong PTT** (hence the name "Lupus Anticoagulant"), but in vivo, it causes **thrombosis** (arterial and venous) [1]. * **Diagnostic Triad:** Look for the "3 Ps": **P**regnancy loss (recurrent), **P**rolonged PTT, and **P**ro-thrombotic state. * **False Positive VDRL:** A classic board-style presentation is a patient with SLE symptoms and a positive VDRL but negative TPHA/FTA-ABS [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Hemodynamic Disorders, Thromboembolic Disease, and Shock, pp. 134-135. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 602-603.

Question 34: All of the following are true EXCEPT:

A. NADPH oxidase acts via superoxide ions
B. Chediak-Higashi syndrome is due to defective phagolysosome formation
C. In Bruton's agammaglobulinemia, opsonization is not affected (Correct Answer)
D. Myeloperoxidase action is mainly due to HOCl

Explanation: ### Explanation **Why Option C is the Correct Answer (The False Statement):** In **Bruton’s Agammaglobulinemia** (X-linked agammaglobulinemia), there is a mutation in the *BTK* gene, leading to a failure of B-cell maturation and a severe deficiency of all classes of immunoglobulins (IgG, IgA, IgM) [4]. **Opsonization is significantly impaired** because IgG is the primary opsonin required for coating bacteria to enhance phagocytosis [2]. Without IgG, patients suffer from recurrent pyogenic infections by encapsulated bacteria (e.g., *S. pneumoniae*, *H. influenzae*) [3]. **Analysis of Other Options:** * **Option A (True):** **NADPH oxidase** (the enzyme deficient in Chronic Granulomatous Disease) is responsible for the "respiratory burst." It converts molecular oxygen into **superoxide ions** ($O_2^-$), the first step in generating reactive oxygen species (ROS). * **Option B (True):** **Chediak-Higashi syndrome** is an autosomal recessive disorder caused by a defect in the *LYST* gene (lysosomal trafficking regulator). This leads to impaired microtubule-dependent **phagolysosome formation**, characterized by giant lysosomal granules in neutrophils [1]. * **Option D (True):** **Myeloperoxidase (MPO)**, found in azurophilic granules, uses hydrogen peroxide and halide ions (like $Cl^-$) to produce **hypochlorous acid (HOCl)**. HOCl (bleach) is the most potent bactericidal agent in the phagocyte's arsenal. **High-Yield Clinical Pearls for NEET-PG:** * **Nitroblue Tetrazolium (NBT) Test:** Used for Chronic Granulomatous Disease (CGD). In CGD, the test is negative (no blue color) because NADPH oxidase is absent. * **Chediak-Higashi Hallmark:** Look for **giant granules** in neutrophils and **partial albinism** in clinical vignettes [1]. * **MPO Deficiency:** Most patients are asymptomatic, but they may have a predisposition to *Candida* infections. * **Opsonins:** The two most important opsonins in the body are **IgG** and **C3b** [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 245-246. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 89-91. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 165-166. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 166-167.

Question 35: Which of the following is a flow cytometric B cell marker?

A. CD 2
B. CD 3
C. CD 7
D. CD 19 (Correct Answer)

Explanation: **Explanation:** Flow cytometry is a vital diagnostic tool in hematopathology used to identify specific cell lineages based on **Cluster of Differentiation (CD)** markers. **Correct Option: D (CD 19)** CD 19 is considered the most reliable and ubiquitous marker for the **B-cell lineage** [2]. It is expressed early in B-cell development (pro-B stage) and persists through all stages of maturation until it is lost during terminal differentiation into plasma cells [2]. Along with **CD 20** and **CD 22**, it is used to identify B-cell lymphomas and leukemias (e.g., B-ALL, CLL) [3]. **Analysis of Incorrect Options:** * **CD 2:** This is a pan-T cell marker and a receptor for sheep red blood cells (associated with the E-rosette test). It is also expressed on Natural Killer (NK) cells. * **CD 3:** This is the most specific marker for **T-cells** [2]. It is physically associated with the T-cell receptor (TCR) complex and is essential for T-cell activation [1]. * **CD 7:** This is one of the earliest markers expressed in T-cell ontogeny. It is often used to identify T-cell acute lymphoblastic leukemia (T-ALL). **High-Yield Clinical Pearls for NEET-PG:** * **Pan-B cell markers:** CD 19, CD 20, CD 22. * **Pan-T cell markers:** CD 2, CD 3, CD 5, CD 7. * **Plasma cell markers:** CD 138 (Syndecan-1) and CD 38. Note that plasma cells are typically **CD 19 negative** [2]. * **NK cell markers:** CD 16 and CD 56. * **Hairy Cell Leukemia:** Characterized by CD 11c, CD 25, and CD 103 [2]. * **Reed-Sternberg Cells (HL):** Classically CD 15+ and CD 30+. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 199-200. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, p. 598. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, p. 602.

Question 36: Changes in which structural level of amyloid protein makes it insoluble?

A. Primary structure
B. Secondary structure (Correct Answer)
C. Tertiary structure
D. Quaternary structure

Explanation: **Explanation:** The fundamental defect in amyloidosis is the **misfolding of proteins**, which transforms normally soluble proteins into insoluble, pathological fibrils [1]. **Why Secondary Structure is Correct:** Amyloidosis is characterized by a transition in the protein's **secondary structure**. Normal proteins (often in $\alpha$-helical or random coil configurations) undergo conformational changes to form **cross-$\beta$-pleated sheets** [1]. In this arrangement, polypeptide chains are folded into sheets that stack perpendicularly to the fibril axis. This specific $\beta$-sheet configuration is highly stable, resistant to proteolysis, and creates the characteristic insolubility that leads to tissue deposition and organ dysfunction [1]. **Why Other Options are Incorrect:** * **Primary Structure:** This refers to the linear sequence of amino acids. While mutations in the primary sequence (e.g., in Transthyretin) can predispose a protein to misfold, the insolubility itself arises only after the folding pattern changes. * **Tertiary & Quaternary Structures:** These refer to the overall 3D folding of a single polypeptide and the arrangement of multiple subunits, respectively. While these levels are altered during the process, the **defining pathognomonic change** that confers the unique physical properties of amyloid (like Congo Red binding) is specifically the $\beta$-pleated secondary structure [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Congo Red Stain:** Amyloid shows **apple-green birefringence** under polarized light due to the $\beta$-pleated sheet structure [1], [2]. * **Morphology:** On H&E, amyloid appears as an extracellular, amorphous, eosinophilic hyaline material. * **Common Types:** **AL** (Light chains, associated with Multiple Myeloma) and **AA** (Serum Amyloid Associated, seen in chronic inflammation like TB or Rheumatoid Arthritis) [1], [2]. * **Diagnosis:** Abdominal fat pad biopsy or rectal biopsy are common screening sites. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 264-269. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 533-534.

Question 37: In which year was HIV discovered?

A. 1983 (Correct Answer)
B. 1979
C. 1969
D. 1990

Explanation: **Explanation:** The Human Immunodeficiency Virus (HIV) was first isolated and identified in **1983** by a team led by **Luc Montagnier** at the Pasteur Institute in France. They initially named the virus Lymphadenopathy-Associated Virus (LAV). Simultaneously, in 1984, Robert Gallo’s team in the USA identified the same virus, calling it HTLV-III. The international committee later standardized the name to HIV in 1986. **Analysis of Options:** * **1983 (Correct):** This marks the definitive discovery of the retrovirus as the causative agent of AIDS, just two years after the first clinical cases were reported. * **1979:** While retrospective studies suggest HIV was circulating in certain populations, the virus had not yet been identified, and the clinical syndrome of AIDS was not yet recognized. * **1969:** This year is often cited in "Patient Zero" theories regarding the early spread of the virus to the Americas, but no medical discovery of the virus occurred during this decade. * **1990:** By this time, HIV had already become a global pandemic, and the first antiretroviral drug, Zidovudine (AZT), had already been approved (1987). **High-Yield Clinical Pearls for NEET-PG:** * **Family:** Retroviridae; **Genus:** Lentivirus [1]. * **First Clinical Report:** June 1981 (CDC report on Pneumocystis pneumonia in MSM). * **Target Cells:** CD4+ T-lymphocytes, macrophages, and dendritic cells [1]. * **Screening Test:** ELISA (High sensitivity) [2]. * **Confirmatory Test:** Western Blot (detects antibodies to p24, gp41, and gp120/160). *Note: Newer algorithms prefer 4th Gen p24 antigen/antibody immunoassays.* [2] * **Monitoring:** Viral load (RT-PCR) is the best predictor of disease progression. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 251-255. [2] Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 169-170.

Question 38: Which of the following statements is NOT true regarding Natural Killer (NK) cells?

A. NK cells are active against malignant cells.
B. NK cells involve MHC antigens for killing micro-organisms. (Correct Answer)
C. NK cells are a first line of defense against viral infections.
D. No prior sensitization is required for NK cell activation.

Explanation: ### Explanation **1. Why Option B is the Correct Answer (The "Not True" Statement):** Natural Killer (NK) cells are unique because they function via **MHC-unrestricted killing**. Unlike Cytotoxic T-cells (CD8+), which require the presentation of antigens on MHC Class I molecules to recognize a target, NK cells are actually **inhibited** by the presence of self-MHC Class I molecules [1]. This is known as the "Missing Self" hypothesis: NK cells possess inhibitory receptors (KIRs) that recognize MHC-I on healthy cells and prevent their destruction [1]. When a virus or tumor downregulates MHC-I expression to evade T-cells, the NK cell loses this inhibitory signal and kills the target. **2. Analysis of Other Options:** * **Option A:** NK cells are a vital component of **immunosurveillance** [2]. They recognize and eliminate stressed, transformed, or malignant cells that have altered surface markers. * **Option C:** They are considered the **first line of defense** against viral infections [2]. They act rapidly (within hours) to control viral replication before the adaptive immune system (T and B cells) is fully primed. * **Option D:** NK cells are part of the **innate immune system** [1]. Unlike the adaptive system, they do not require prior exposure (sensitization) or clonal expansion to function; they are "born to kill." **3. NEET-PG High-Yield Clinical Pearls:** * **Origin:** Derived from Common Lymphoid Progenitors (CLP) but lack TCR and CD3. * **Markers:** Characteristically express **CD16** (Fc\u03b3RIII, used for Antibody-Dependent Cellular Cytotoxicity - ADCC) and **CD56** (NCAM). * **Effector Mechanism:** They induce apoptosis via **Perforins and Granzymes** or the Fas/FasL pathway. * **Cytokine Production:** They are a major source of **IFN-\u03b3**, which activates macrophages [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 200-201. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 164-165.

Question 39: Henoch-Schonlein purpura is characterized by deposition of which immunoglobulin?

A. IgG
B. IgA (Correct Answer)
C. IgM
D. IgE

Explanation: **Explanation:** **Henoch-Schönlein Purpura (HSP)**, also known as **IgA Vasculitis**, is a systemic small-vessel vasculitis characterized by the deposition of immune complexes containing **IgA1** [1]. 1. **Why IgA is Correct:** The hallmark of HSP is the deposition of IgA-dominant immune complexes in the walls of small vessels (capillaries, venules, and arterioles). This triggers an inflammatory response (leukocytoclastic vasculitis). The disease often follows an upper respiratory tract infection, which stimulates mucosal IgA production. On immunofluorescence, these deposits are found in the skin (dermal capillaries), kidneys (mesangium), and GI tract. 2. **Why Other Options are Incorrect:** * **IgG & IgM:** While these may occasionally be found as secondary components in the immune complexes, they are not the primary diagnostic or pathogenic drivers of HSP. IgG is more characteristic of conditions like SLE or Polyarteritis Nodosa. * **IgE:** This immunoglobulin is associated with Type I hypersensitivity (allergic) reactions and parasitic infections, not the Type III hypersensitivity (immune-complex mediated) mechanism seen in HSP [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Tetrad:** Palpable purpura (usually on buttocks/legs), Arthralgia, Abdominal pain (may lead to intussusception), and Renal involvement (HSP Nephritis). * **Renal Pathology:** HSP Nephritis is histologically indistinguishable from **IgA Nephropathy (Berger’s Disease)**; the primary difference is that HSP is a systemic multi-organ disease [1]. * **Diagnosis:** Primarily clinical; skin biopsy shows **leukocytoclastic vasculitis** with IgA deposits on immunofluorescence. * **Demographics:** It is the most common vasculitis in children. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 535-536. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 214-215.

Question 40: CD8 is a marker for which of the following cell types?

A. T-cell (Correct Answer)
B. B-cell
C. NK-cell
D. Macrophage

Explanation: **Explanation:** **CD8 (Cluster of Differentiation 8)** is a transmembrane glycoprotein that serves as a co-receptor for the **T-cell receptor (TCR)** [3]. It is specifically expressed on **Cytotoxic T-cells** [1]. These cells recognize antigens presented by **MHC Class I** molecules (the "Rule of 8": 8 x 1 = 8) [2]. Upon activation, CD8+ T-cells induce apoptosis in virally infected or tumor cells via perforins and granzymes. **Analysis of Options:** * **A. T-cell (Correct):** CD8 is the hallmark marker for cytotoxic T-lymphocytes [3]. While CD3 is the pan-T-cell marker, CD4 and CD8 define the two major functional subsets of T-cells. * **B. B-cell:** B-cells are characterized by markers such as **CD19, CD20, and CD21**. They do not express CD8. * **C. NK-cell:** While some NK cells can express CD8, their primary diagnostic markers are **CD16** (Fc̲̲̲̲̳̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̲̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̄̑RIII) and **CD56** (NCAM). In the context of standard medical examinations, CD8 is classically associated with T-cells. * **D. Macrophage:** Macrophages are identified by markers like **CD14, CD68, and CD11b**. They act as professional antigen-presenting cells (APCs) rather than cytotoxic effectors. **High-Yield Clinical Pearls for NEET-PG:** * **Pan-T-cell marker:** CD3. * **Helper T-cells:** CD4+ (binds to MHC Class II). * **Mantle Cell Lymphoma:** Characterized by CD5+ B-cells. * **Hairy Cell Leukemia:** Characterized by CD103, CD11c, and CD25. * **Flow Cytometry:** The gold standard technique used to identify these CD markers for diagnosing leukemias and lymphomas. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 240. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 318-319. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 198-199.

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