Immunopathology — MCQs

A 14-month-old child has had multiple infections since birth, including pneumonia with Pseudomonas aeruginosa, adenovirus, and Aspergillus fumigatus; diarrhea with Isospora belli; otitis media with Haemophilus influenzae; and urinary tract infection with Candida albicans. Laboratory studies show hemoglobin, 13.2 g/dL; hematocrit, 39.7%; platelet count, 239,100/mm3; and WBC count, 3450/mm3 with 85% segmented neutrophils, 6% bands, 2% lymphocytes, and 7% monocytes. Serum immunoglobulin levels are IgG, 118 mg/dL; IgM, 14 mg/dL; and IgA, 23 mg/dL. The child dies of pneumonia. At autopsy, a hypoplastic thymus, small lymph nodes that lack germinal centers, and scant gut-associated lymphoid tissue are seen. Which of the following is the most likely cause of this disease?

Q137Easy

Which Human Leukocyte Antigen (HLA) class is associated with CD4?

Q138Easy

What condition is indicated by a lymphocyte phenotype test?

Q139Easy

All of the following are involved in graft-versus-host disease except?

Q140Medium

Hypersensitivity pneumonitis is classically described as which type of hypersensitivity reaction?

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 131: Which of the following is characteristic of Graft-versus-Host Disease (GVHD)?

A. Associated with solid organ transplantation
B. Graft must contain immunocompetent T-cells
C. It is seen in immunosuppressed persons
D. All of the above (Correct Answer)

Explanation: **Explanation:** Graft-versus-Host Disease (GVHD) occurs when immunologically competent cells (or their precursors) are transplanted into immunologically compromised recipients, and the transferred cells recognize the recipient's HLA antigens as foreign, leading to a systemic attack [1]. **Why "All of the Above" is correct:** * **Option A (Associated with solid organ transplantation):** While most commonly associated with **Hematopoietic Stem Cell Transplant (HSCT)**, GVHD can occur in solid organ transplants rich in lymphoid tissue, such as the **liver** or small bowel, and even following non-irradiated blood transfusions [4]. * **Option B (Graft must contain immunocompetent T-cells):** This is a fundamental requirement (Billingham’s Criteria). The donor T-cells (CD4+ and CD8+) must be functional to recognize and mount an immune response against the host tissues [1]. * **Option C (Seen in immunosuppressed persons):** For GVHD to develop, the host must be unable to reject the graft. This occurs in patients who are immunocompromised due to chemotherapy, irradiation, or primary immunodeficiency [3]. **High-Yield Clinical Pearls for NEET-PG:** 1. **Billingham’s Criteria:** (1) Graft must have immunocompetent cells, (2) Host must possess antigens lacking in the donor, (3) Host must be unable to mount an effective immune response against the graft. 2. **Acute GVHD (within 100 days):** Characterized by the "Triad" of **Dermatitis** (maculopapular rash), **Enteritis** (diarrhea), and **Hepatitis** (jaundice/elevated bilirubin) [1]. 3. **Chronic GVHD (after 100 days):** Mimics autoimmune disorders like **Systemic Sclerosis** (skin fibrosis) or Sjögren’s syndrome [2]. 4. **Graft-versus-Leukemia (GVL) effect:** In HSCT, a mild degree of GVHD is often beneficial as the donor T-cells also attack residual leukemic cells in the recipient [3]. **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. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 244-245. [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. 181-182.

Question 132: Pinch purpura is seen in which of the following conditions?

A. Primary systemic amyloidosis (Correct Answer)
B. Vitamin C deficiency
C. Purpura fulminans
D. Kawasaki disease

Explanation: **Explanation:** **Pinch purpura** (also known as post-procedural or traumatic purpura) is a classic dermatological manifestation of **Primary Systemic Amyloidosis (AL type)** [1]. **Why it occurs:** In AL amyloidosis, light chain amyloid fibrils deposit within the walls of small blood vessels (capillaries and arterioles) [1]. This infiltration leads to increased **vascular fragility** [2]. Even minor trauma, such as rubbing or pinching the skin (especially in periorbital areas), causes the weakened vessels to rupture, resulting in characteristic ecchymoses or purpura. This is often referred to as the "Raccoon eye" sign when seen around the orbits. **Analysis of Incorrect Options:** * **Vitamin C deficiency (Scurvy):** Characterized by perifollicular hemorrhages, corkscrew hairs, and bleeding gums due to defective collagen synthesis, not amyloid deposition. * **Purpura fulminans:** An acute, often fatal thrombotic disorder associated with Protein C or S deficiency or severe sepsis (e.g., Meningococcemia), presenting with large areas of skin necrosis and DIC. * **Kawasaki disease:** A medium-vessel vasculitis in children characterized by "CRASH and Burn" symptoms (Conjunctivitis, Rash, Adenopathy, Strawberry tongue, Hand/foot edema, and high Fever). **High-Yield Clinical Pearls for NEET-PG:** * **Macroglossia + Nephrotic Syndrome + Pinch Purpura** = Classic triad for Primary Amyloidosis [3]. * **Diagnosis:** Congo Red stain showing **apple-green birefringence** under polarized light [1]. * **Factor X Deficiency:** Amyloid fibrils can adsorb Factor X, leading to an acquired bleeding diathesis in these patients. **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] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, pp. 580-581. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 618-619.

Question 133: Which cells mediate Type IV hypersensitivity reactions?

A. B cells
B. Active T cells (Correct Answer)
C. NK cells
D. Plasma cells

Explanation: **Explanation:** Type IV hypersensitivity is unique because it is the only hypersensitivity reaction that is **cell-mediated** rather than antibody-mediated. It is primarily driven by **Active T cells** [1] (specifically CD4+ Th1, Th17, and CD8+ T cells) [2]. Upon exposure to an antigen, sensitized T cells release cytokines (like IFN-γ and IL-2) [1] that activate macrophages or directly induce cytotoxicity. Because this process requires the recruitment and activation of cells, the clinical manifestation is typically delayed (occurring 48–72 hours after exposure) [2], leading to its common name: **Delayed-Type Hypersensitivity (DTH).** **Analysis of Incorrect Options:** * **B cells (A) & Plasma cells (D):** These are involved in humoral immunity. They produce antibodies (IgE, IgG, IgM) which mediate Type I, II, and III hypersensitivity reactions. Type IV does not involve antibodies. * **NK cells (C):** While NK cells are part of the innate immune system and can kill virally infected or tumor cells, they are not the primary mediators of the specific, memory-driven response seen in Type IV hypersensitivity. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Examples:** Mantoux (Tuberculin) test [3], Contact Dermatitis (e.g., Poison Ivy, Nickel allergy) [3], Graft rejection (Acute/Chronic) [4], and Granuloma formation (Sarcoidosis, TB). * **Key Cytokine:** **IFN-γ** is the most important cytokine for macrophage activation in DTH [1]. * **Histology:** Look for "perivascular cuffing" by lymphocytes and macrophages. * **Memory Tip:** Use the mnemonic **"ACID"** for Hypersensitivity types: **A**naphylactic (I), **C**ytotoxic (II), **I**mmune-Complex (III), and **D**elayed/Cell-mediated (IV). **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. 173-174. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 216-218. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 218-219. [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. 174-175.

Question 134: C-ANCA positivity indicates antibody formation against which of the following?

A. Proteinase 3 (Correct Answer)
B. Myeloperoxidase
C. Cytoplasmic antinuclear antibody
D. Anti centromere antibody

Explanation: **Explanation:** The diagnosis of systemic vasculitis often relies on the detection of **Antineutrophil Cytoplasmic Antibodies (ANCA)** via immunofluorescence [1]. **1. Why Proteinase 3 (PR3) is correct:** ANCA patterns are defined by their staining characteristics. **C-ANCA (Cytoplasmic ANCA)** shows a diffuse granular staining pattern throughout the cytoplasm of neutrophils. This pattern is specifically associated with antibodies directed against **Proteinase 3 (PR3)**, a serine protease found in the primary granules of neutrophils. It is a highly specific marker for **Granulomatosis with Polyangiitis (GPA)**, formerly known as Wegener’s Granulomatosis [1]. **2. Analysis of Incorrect Options:** * **B. Myeloperoxidase (MPO):** Antibodies against MPO result in a **P-ANCA (Perinuclear)** pattern [1]. This is typically seen in Microscopic Polyangiitis (MPA), Churg-Strauss Syndrome (EGPA), and Primary Sclerosing Cholangitis. * **C. Cytoplasmic antinuclear antibody:** This is a misnomer. While ANCA stands for Antineutrophil Cytoplasmic Antibody, "antinuclear" refers to ANA, which targets antigens within the cell nucleus (common in SLE). * **D. Anti-centromere antibody:** This is a specific ANA pattern associated with **CREST syndrome** (Limited Systemic Sclerosis), not vasculitis. **High-Yield Clinical Pearls for NEET-PG:** * **C-ANCA / PR3-ANCA:** Best marker for **Granulomatosis with Polyangiitis (GPA)** [1]. Antibody titers often correlate with disease activity. * **P-ANCA / MPO-ANCA:** Associated with the "P" diseases: **P**ause-immune crescentic glomerulonephritis, **P**olyangiitis (Microscopic), and Churg-Strauss [1]. * **Triad of GPA:** Necrotizing granulomas of the respiratory tract, necrotizing vasculitis, and focal necrotizing glomerulonephritis. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 917-918.

Question 135: Which of the following is an example of Type 3 hypersensitivity?

A. Asthma
B. Contact dermatitis
C. SLE (Correct Answer)
D. AIHA

Explanation: **Explanation:** **Type 3 Hypersensitivity** is an **Immune-Complex Mediated** reaction [1], [4]. It occurs when antigen-antibody complexes (usually IgG or IgM) are formed in the circulation and deposited in various tissues (e.g., kidneys, joints, blood vessels) [4]. This deposition triggers the classical complement pathway, leading to neutrophil recruitment and subsequent tissue damage [3]. * **Systemic Lupus Erythematosus (SLE)** is the classic example of Type 3 hypersensitivity [1]. In SLE, autoantibodies are directed against nuclear antigens (like dsDNA). These circulating immune complexes deposit in the glomeruli (causing glomerulonephritis) and small blood vessels (causing vasculitis) [1], [2]. **Analysis of Incorrect Options:** * **A. Asthma:** This is a **Type 1 (Immediate)** hypersensitivity reaction. It is mediated by IgE antibodies binding to mast cells, leading to the release of histamine and leukotrienes upon allergen exposure. * **B. Contact Dermatitis:** This is a **Type 4 (Delayed-type)** hypersensitivity reaction. It is T-cell mediated (CD4+ and CD8+ cells) and typically occurs 48–72 hours after exposure to antigens like nickel or poison ivy. * **D. Autoimmune Hemolytic Anemia (AIHA):** This is a **Type 2 (Antibody-mediated)** hypersensitivity reaction. Antibodies (IgG/IgM) bind directly to antigens on the surface of red blood cells, leading to their destruction via opsonization or complement-mediated lysis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity (ACID):** **A**naphylactic (Type 1), **C**ytotoxic (Type 2), **I**mmune-Complex (Type 3), **D**elayed-type (Type 4). * **Other Type 3 Examples:** Post-streptococcal glomerulonephritis (PSGN), Serum Sickness, Arthus Reaction, and Rheumatoid Arthritis [1], [3]. * **Key Mediator:** Complement fragment **C5a** is crucial in Type 3 reactions for attracting neutrophils that cause tissue injury [3]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 215-216. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 514-515. [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. 172-173. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 214-215.

Question 136: A 14-month-old child has had multiple infections since birth, including pneumonia with Pseudomonas aeruginosa, adenovirus, and Aspergillus fumigatus; diarrhea with Isospora belli; otitis media with Haemophilus influenzae; and urinary tract infection with Candida albicans. Laboratory studies show hemoglobin, 13.2 g/dL; hematocrit, 39.7%; platelet count, 239,100/mm3; and WBC count, 3450/mm3 with 85% segmented neutrophils, 6% bands, 2% lymphocytes, and 7% monocytes. Serum immunoglobulin levels are IgG, 118 mg/dL; IgM, 14 mg/dL; and IgA, 23 mg/dL. The child dies of pneumonia. At autopsy, a hypoplastic thymus, small lymph nodes that lack germinal centers, and scant gut-associated lymphoid tissue are seen. Which of the following is the most likely cause of this disease?

A. Abnormal CD40 ligand
B. Adenosine deaminase deficiency (Correct Answer)
C. BTK gene mutation
D. Chromosome 22q11 deletion

Explanation: ### Explanation The clinical presentation describes a classic case of **Severe Combined Immunodeficiency (SCID)** [4]. The child has suffered from a broad spectrum of infections (bacterial, viral, fungal, and protozoal) starting in infancy, indicating a profound defect in both humoral (B-cell) and cell-mediated (T-cell) immunity [3]. **1. Why Adenosine Deaminase (ADA) Deficiency is Correct:** ADA deficiency is the second most common cause of SCID (autosomal recessive). ADA is essential for the breakdown of adenosine; its deficiency leads to the accumulation of toxic metabolites (deoxyadenosine and dATP) within lymphocytes. These metabolites are lymphotoxic, leading to a **profound decrease in both T-cells and B-cells**. This matches the laboratory findings: * **Lymphopenia:** Only 2% lymphocytes (Normal: ~20-40%). * **Pan-hypogammaglobulinemia:** Very low IgG, IgM, and IgA. * **Morphology:** Hypoplastic thymus (T-cell defect) and absent germinal centers/Peyer’s patches (B-cell defect). **2. Why Other Options are Incorrect:** * **Abnormal CD40 Ligand:** Causes **Hyper-IgM Syndrome**. While it leads to pyogenic infections, it typically presents with normal or elevated IgM and normal T-cell counts. * **BTK Gene Mutation:** Causes **X-linked Agammaglobulinemia (Bruton’s)**. This is a pure B-cell defect [2]. T-cell immunity and the thymus remain normal [1]; therefore, patients do not typically succumb to opportunistic infections like *Aspergillus* or *Isospora*. * **Chromosome 22q11 Deletion:** Causes **DiGeorge Syndrome**. While it presents with a hypoplastic thymus and T-cell deficiency, B-cell numbers and immunoglobulin levels are often near normal [4]. It is also associated with hypocalcemia and congenital heart defects, which are absent here [4]. **Clinical Pearls for NEET-PG:** * **SCID Inheritance:** Most common is **X-linked** (IL-2RG mutation); second most common is **ADA deficiency** (Autosomal Recessive). * **Diagnosis:** Low TRECs (T-cell receptor excision circles) on newborn screening. * **Management:** SCID is a pediatric emergency; definitive treatment is **Hematopoietic Stem Cell Transplant (HSCT)** [4]. ADA deficiency can also be treated with gene therapy or enzyme replacement. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 248-249. [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. 166-167. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 246-247. [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. 167-168.

Question 137: Which Human Leukocyte Antigen (HLA) class is associated with CD4?

A. HLA class I
B. HLA class II (Correct Answer)
C. HLA class III
D. All of them

Explanation: **Explanation:** The interaction between T-lymphocytes and Antigen-Presenting Cells (APCs) is governed by the principle of **MHC Restriction**. **Why HLA Class II is correct:** HLA Class II molecules (HLA-DR, DQ, DP) are primarily expressed on professional APCs (macrophages, B-cells, and dendritic cells) [2]. The **CD4 molecule** acts as a co-receptor that specifically binds to the invariant portion of the **HLA Class II** molecule [1]. This interaction is essential for the activation of Helper T-cells, which then orchestrate the immune response against exogenous antigens [3]. **Why other options are incorrect:** * **HLA Class I:** These molecules (HLA-A, B, C) are expressed on all nucleated cells and platelets [2]. They associate with **CD8+ Cytotoxic T-cells** [1]. A helpful mnemonic for NEET-PG is the **"Rule of 8"**: (Class I × CD8 = 8) and (Class II × CD4 = 8). * **HLA Class III:** These genes encode for components of the complement system (C2, C4), cytokines (TNF-́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̑́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́̂-alpha, TNF-̂-beta), and heat shock proteins. They do not interact with CD4 or CD8 T-cell receptors. * **All of them:** Incorrect, as the binding is highly specific to ensure the correct immune pathway is triggered. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** HLA Class I has one heavy chain and a **̂̂-microglobulin**; HLA Class II consists of two heavy chains (́-alpha and ̂-beta) [2]. * **Endosomal Pathway:** HLA Class II molecules process **exogenous** antigens (e.g., bacteria), whereas Class I molecules process **endogenous** antigens (e.g., viruses, tumor proteins) [3]. * **Key Association:** HLA-B27 is strongly linked to Seronegative Spondyloarthropathies (e.g., Ankylosing Spondylitis). **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] 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. 156-157. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 203-204.

Question 138: What condition is indicated by a lymphocyte phenotype test?

A. Agammaglobulinemia
B. Severe Combined Immunodeficiency (SCID) (Correct Answer)
C. Sepsis
D. Acute leukemia

Explanation: **Explanation:** **Lymphocyte Phenotyping** (using Flow Cytometry) is the gold standard for diagnosing primary immunodeficiency disorders [2]. It identifies specific cell surface markers (CD markers) to quantify T cells (CD3+), B cells (CD19/20+), and Natural Killer cells (CD16/56+). **Why SCID is the correct answer:** Severe Combined Immunodeficiency (SCID) is characterized by a profound defect in both humoral and cellular immunity [3]. Phenotyping is essential to classify SCID based on which cell lines are absent (e.g., T-B+NK- or T-B-NK+). Since SCID involves a fundamental lack of lymphocyte subsets, phenotyping is the definitive diagnostic step to assess the immune repertoire [3]. **Analysis of Incorrect Options:** * **Agammaglobulinemia:** While phenotyping shows absent B cells (CD19+), the primary screening test is measuring serum immunoglobulin levels (IgG, IgA, IgM), which will be significantly low [4]. * **Sepsis:** This is a clinical diagnosis based on systemic inflammatory response to infection. While a CBC might show leukocytosis or lymphopenia, lymphocyte phenotyping is not a standard diagnostic tool for acute sepsis. * **Acute Leukemia:** Diagnosis primarily relies on bone marrow aspiration, biopsy, and cytogenetics. While flow cytometry is used for "immunophenotyping" to identify the lineage of blasts, the term "lymphocyte phenotype test" in a general pathology context specifically refers to evaluating immune deficiency states [1]. **NEET-PG High-Yield Pearls:** * **Most common SCID:** X-linked SCID (Mutation in the γ-chain of IL-2 receptor) [3]. * **Most common enzyme deficiency in SCID:** Adenosine Deaminase (ADA) deficiency (Autosomal Recessive) [3]. * **TRECs (T-cell Receptor Excision Circles):** Used in newborn screening for SCID; low levels indicate a lack of T-cell neogenesis. * **Flow Cytometry Markers:** CD3 (Total T cells), CD4 (Helper), CD8 (Cytotoxic), CD19/20 (B cells), CD16/56 (NK cells). **References:** [1] 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, pp. 596-598. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 246-247. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 247-248. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 248-249.

Question 139: All of the following are involved in graft-versus-host disease except?

A. Kidney (Correct Answer)
B. Liver
C. Stomach
D. Skin

Explanation: **Explanation:** **Graft-versus-host disease (GVHD)** occurs when immunologically competent donor T-cells (the graft) recognize the recipient’s (the host) HLA antigens as foreign and mount an immune attack. This typically occurs in the setting of hematopoietic stem cell transplantation or solid organ transplants rich in lymphoid tissue. **Why Kidney is the correct answer:** The kidney is **not** a primary target organ for GVHD. While the kidney can be affected by drug toxicity (e.g., Cyclosporine) or transplant rejection (Host-vs-Graft), it does not possess the specific epithelial microenvironment or cytokine milieu that triggers the donor T-cell mediated destruction characteristic of GVHD [1]. **Why other options are incorrect:** GVHD primarily targets epithelial surfaces. The "classic triad" of organs involved includes: * **Skin (Option D):** Usually the first organ involved, presenting as a maculopapular rash (acute) or scleroderma-like changes (chronic). * **Liver (Option B):** Presents with cholestatic jaundice due to the destruction of small bile ducts and elevated alkaline phosphatase. * **Gastrointestinal Tract (Option C):** The **stomach** and intestines are frequently involved, leading to mucosal ulcerations, profuse watery diarrhea, and abdominal pain. **High-Yield Clinical Pearls for NEET-PG:** * **Prerequisites (Billingham’s Criteria):** 1. Graft must contain immunocompetent cells; 2. Host must be immunocompromised; 3. Host must express antigens foreign to the donor. * **Acute vs. Chronic:** Acute GVHD occurs within 100 days; Chronic GVHD occurs after 100 days and mimics autoimmune diseases (e.g., Sjogren’s syndrome, Scleroderma). * **Target Cells:** The primary effector cells are **Donor CD4+ and CD8+ T-cells**. * **Graft-versus-Leukemia (GVL) effect:** A beneficial aspect of GVHD where donor cells also attack residual leukemic cells in the recipient. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 239-242.

Question 140: Hypersensitivity pneumonitis is classically described as which type of hypersensitivity reaction?

A. Type I hypersensitivity reaction
B. Type II hypersensitivity reaction
C. Type III (immune complex) hypersensitivity
D. Type IV (cell mediated) hypersensitivity (Correct Answer)

Explanation: **Explanation:** **Hypersensitivity Pneumonitis (HP)**, also known as extrinsic allergic alveolitis, is an immunologic lung disease caused by an exaggerated immune response to inhaled organic antigens (e.g., Farmer’s lung, Bird fancier’s lung) [1], [2]. **Why Type IV is the Correct Answer:** Classically, the chronic and subacute phases of HP are driven by **Type IV (Delayed-type) hypersensitivity** [1]. This involves T-cell mediated responses (CD4+ and CD8+ lymphocytes) leading to cytokine release, macrophage activation, and the hallmark pathological finding: **non-caseating granulomas** in the interstitium [1]. While Type III reactions occur in the acute phase, the definitive diagnostic and pathological description for exams is Type IV. **Why Other Options are Incorrect:** * **Type I:** This is IgE-mediated (e.g., Asthma). HP does not involve IgE or eosinophilia. * **Type II:** This involves cytotoxic antibodies against fixed tissue antigens (e.g., Goodpasture syndrome). HP involves inhaled environmental antigens, not self-antigens. * **Type III:** While immune complexes (Type III) are present in the acute phase of HP (causing neutrophilic infiltration), the **classical** description and the formation of granulomas are attributed to Type IV [1]. **NEET-PG High-Yield Pearls:** * **Histology Triad:** Interstitial pneumonitis, non-caseating granulomas, and peribronchiolar fibrosis [1]. * **Common Examples:** Farmer’s Lung (Actinomycetes in moldy hay), Bagassosis (moldy sugar cane), Silo filler's disease (nitrogen dioxide) [2]. * **Key Difference:** Unlike Sarcoidosis, HP granulomas are usually "loose" or ill-defined and located near bronchioles. * **PFT:** Shows a **Restrictive pattern** with decreased DLCO. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 701-702. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Respiratory Tract Disease, pp. 332-333.

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