Immunopathology — MCQs

A 29-year-old woman has had increasing weakness over the past year, with difficulty climbing a single flight of stairs and frequent muscle soreness. She has little difficulty writing or typing, but has experienced increasing difficulty swallowing over the past 3 months and chest pain for the past week. On physical examination, she is afebrile with a blood pressure of 115/75 mm Hg. Muscle strength is 4/5 in all extremities, with no rashes present. She has 2+ pitting edema to the knees and rales are auscultated over the lower lung fields. Laboratory studies show a serum creatine kinase level of 458 U/L and Jo-1 antibodies. Which of the following additional complications of her disease is she most likely to have?

Q195Medium

All of the following immunodeficiency diseases are due to chromosomal instability or defective repair of genes, except:

Q196Easy

Henoch-Schönlein purpura is characterized by all of the following except?

Q197Easy

Thymoma is associated with which of the following conditions?

Q198Easy

In Wegener's granulomatosis, cytoplasmic anti-neutrophilic antibodies are directed against which of the following?

Q199Easy

Acute Graft-versus-Host Disease (GVHD) is primarily caused by which type of lymphocyte?

Q200Medium

Which of the following organs can cause an antigen reaction when exposed in self-blood?

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 191: Angioedema is seen due to deficiency of?

A. Angiotensin converting enzyme
B. C1 esterase inhibitor (Correct Answer)
C. Histamine
D. Angiotensin 1

Explanation: **Explanation:** **1. Why C1 esterase inhibitor is correct:** Hereditary Angioedema (HAE) is an autosomal dominant disorder caused by a deficiency or dysfunction of the **C1 esterase inhibitor (C1-INH)**. * **Mechanism:** C1-INH is a serine protease inhibitor that normally regulates the classical complement pathway and the kinin system. It inhibits **Kallikrein** and **Factor XIIa**. * When C1-INH is deficient, there is uncontrolled activation of the kinin cascade, leading to excessive production of **Bradykinin** [1]. * Bradykinin increases vascular permeability, resulting in the characteristic non-pitting mucosal and subcutaneous edema (angioedema) without urticaria or pruritus [1]. **2. Why other options are incorrect:** * **Angiotensin-converting enzyme (ACE):** While ACE inhibitors (drugs) are a common *cause* of acquired angioedema (by preventing bradykinin breakdown), the enzyme deficiency itself is not the cause. * **Histamine:** Histamine is involved in allergic (Type I hypersensitivity) angioedema, which is usually accompanied by hives/urticaria. However, the classic "deficiency" associated with hereditary angioedema specifically involves C1-INH. * **Angiotensin 1:** This is a precursor in the RAAS pathway and has no direct role in the pathogenesis of angioedema. **3. NEET-PG High-Yield Pearls:** * **Diagnosis:** Screen with **C4 levels** (always low during attacks). Definitive diagnosis is made by measuring C1-INH levels/function. * **Clinical Presentation:** Recurrent episodes of swelling (face, larynx, GI tract). Laryngeal edema is life-threatening. * **Treatment:** Acute attacks are treated with **C1-INH concentrate**, **Ecallantide** (kallikrein inhibitor), or **Icatibant** (bradykinin B2 receptor antagonist). * **Key Distinction:** Hereditary angioedema is characterized by the **absence of urticaria and itching**, distinguishing it from IgE-mediated allergic reactions. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 100-101.

Question 192: Which of the following is NOT a feature of DiGeorge Syndrome?

A. Caused due to deletion of the long arm of Chromosome 22
B. Featured with cleft palate
C. Associated with cardiac septal defects
D. Features include hypercalcemia (Correct Answer)

Explanation: ### **Explanation** DiGeorge Syndrome (DGS) is a primary immunodeficiency disorder resulting from the **maldevelopment of the 3rd and 4th pharyngeal pouches**. This leads to a classic triad of thymic hypoplasia, parathyroid hypoplasia, and congenital heart disease [1]. **Why Option D is the Correct Answer:** In DiGeorge Syndrome, the failure of the 3rd and 4th pharyngeal pouches leads to **parathyroid gland hypoplasia** [2]. This results in a deficiency of Parathyroid Hormone (PTH), leading to **hypocalcemia** (low calcium levels), not hypercalcemia. Hypocalcemia often presents in the neonatal period as tetany or seizures [2]. **Analysis of Incorrect Options:** * **Option A:** DGS is most commonly caused by a **microdeletion at chromosome 22q11.2** [1]. This is a high-yield genetic association often tested via FISH (Fluorescence In Situ Hybridization) [3]. * **Option B:** Facial dysmorphism is a hallmark of the 22q11 deletion syndrome spectrum (CATCH-22) [1]. This includes midline defects like **cleft palate** and a bifid uvula. * **Option C:** **Congenital heart defects**, particularly conotruncal anomalies (e.g., Tetralogy of Fallot, Persistent Truncus Arteriosus, and interrupted aortic arch), are frequently associated with this syndrome [1]. --- ### **Clinical Pearls for NEET-PG** * **Mnemonic: CATCH-22** * **C** – Cardiac defects * **A** – Abnormal facies * **T** – Thymic hypoplasia * **C** – Cleft palate * **H** – **Hypocalcemia** * **22** – Chromosome 22q11 deletion * **Immunology:** Patients have a deficiency in cell-mediated immunity (T-cells) but usually have normal B-cell counts and immunoglobulin levels. * **Radiology:** Look for the **absence of a thymic shadow** on a neonatal chest X-ray. **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. 167-168. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1107-1108. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 173.

Question 193: Which antibody is primarily responsible for protecting against intestinal infections?

A. Immunoglobulin A (IgA) (Correct Answer)
B. Immunoglobulin M (IgM)
C. Immunoglobulin G (IgG)
D. Immunoglobulin E (IgE)

Explanation: **Explanation:** The correct answer is **Immunoglobulin A (IgA)**. IgA is the predominant antibody found in mucosal secretions, including the gastrointestinal, respiratory, and genitourinary tracts. In the intestines, it exists primarily as **Secretory IgA (sIgA)**, a dimer held together by a **J-chain** and protected from enzymatic degradation by a **secretory component**. Its primary role is "immune exclusion"—preventing the attachment of pathogens (bacteria and viruses) to the mucosal epithelium, thereby neutralizing them before they can invade the systemic circulation. **Analysis of Incorrect Options:** * **Immunoglobulin M (IgM):** This is the first antibody produced in a primary immune response [1]. While it can be secreted across mucosa (also using a J-chain), it is primarily a pentameric intravascular antibody and not the chief mucosal defender [1]. * **Immunoglobulin G (IgG):** This is the most abundant antibody in the serum and provides long-term immunity and opsonization [2]. It crosses the placenta but does not have a specialized transport mechanism for mucosal surfaces. * **Immunoglobulin E (IgE):** This antibody is primarily involved in Type I hypersensitivity reactions (allergy) and defense against helminthic (parasitic) infections by activating mast cells and eosinophils. **NEET-PG High-Yield Pearls:** * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent sinopulmonary and GI infections (e.g., *Giardia lamblia*) [3]. * **Breast Milk:** IgA is the most abundant immunoglobulin in colostrum, providing passive mucosal immunity to the neonate. * **Peyer’s Patches:** These are the lymphoid tissues in the ileum where B-cells undergo class-switching to become IgA-producing plasma cells. **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. 154-155. [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. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 249-250.

Question 194: A 29-year-old woman has had increasing weakness over the past year, with difficulty climbing a single flight of stairs and frequent muscle soreness. She has little difficulty writing or typing, but has experienced increasing difficulty swallowing over the past 3 months and chest pain for the past week. On physical examination, she is afebrile with a blood pressure of 115/75 mm Hg. Muscle strength is 4/5 in all extremities, with no rashes present. She has 2+ pitting edema to the knees and rales are auscultated over the lower lung fields. Laboratory studies show a serum creatine kinase level of 458 U/L and Jo-1 antibodies. Which of the following additional complications of her disease is she most likely to have?

A. Bony ankylosis
B. Myocarditis (Correct Answer)
C. Pericarditis
D. Sclerodactyly

Explanation: ### **Explanation** **Diagnosis: Polymyositis (PM)** The patient presents with progressive proximal muscle weakness (difficulty climbing stairs), dysphagia, elevated Creatine Kinase (CK), and **Anti-Jo-1 antibodies**. The absence of a rash (no heliotrope or Gottron papules) distinguishes this as Polymyositis rather than Dermatomyositis [1]. **1. Why Myocarditis is Correct:** Polymyositis is a systemic autoimmune disorder where CD8+ T-cells infiltrate the endomysium [2]. While it primarily affects skeletal muscle, the inflammatory process can involve the **cardiac muscle (myocarditis)**. This patient’s presentation of chest pain, rales, and pitting edema (signs of congestive heart failure) strongly suggests cardiac involvement. Myocarditis or conduction disturbances are significant causes of morbidity in PM/DM patients. **2. Why Incorrect Options are Wrong:** * **A. Bony ankylosis:** This is a hallmark of Ankylosing Spondylitis or severe Rheumatoid Arthritis, not inflammatory myopathies. * **C. Pericarditis:** While possible in systemic lupus erythematosus (SLE), it is much less common than myocarditis in Polymyositis. * **D. Sclerodactyly:** This is a feature of Systemic Sclerosis (Scleroderma) or CREST syndrome, characterized by skin thickening and tightening [3]. **3. NEET-PG High-Yield Pearls:** * **Anti-Jo-1 (Histidyl-tRNA synthetase):** Most common antibody in PM; strongly associated with **Interstitial Lung Disease (ILD)**, Raynaud’s phenomenon, and "Mechanic's hands" [1]. * **Anti-Mi-2:** Highly specific for Dermatomyositis (good prognosis). * **Anti-SRP:** Associated with severe, necrotizing myopathy and poor prognosis. * **Biopsy Findings:** PM shows **endomysial** inflammation (CD8+ T-cells); DM shows **perimysial/perivascular** inflammation (CD4+ T-cells) and perifascicular atrophy [2]. * **Malignancy:** Both PM and DM (especially DM) carry an increased risk of visceral malignancies (lung, ovary, GI). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1240-1241. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1241-1242. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Osteoarticular And Connective Tissue Disease, pp. 689-690.

Question 195: All of the following immunodeficiency diseases are due to chromosomal instability or defective repair of genes, except:

A. Ataxia telangiectasia
B. Bloom syndrome
C. Seckel syndrome
D. Chediak-Higashi syndrome (Correct Answer)

Explanation: The core concept tested here is the classification of primary immunodeficiencies based on their molecular pathogenesis. **Chediak-Higashi Syndrome (Correct Answer):** This is a **lysosomal trafficking defect**, not a DNA repair disorder. It is caused by a mutation in the **LYST (CHS1) gene**, which regulates intracellular protein trafficking [1]. This leads to the formation of giant peroxidase-positive inclusions in neutrophils and defective degranulation. It is characterized by the triad of partial oculocutaneous albinism, recurrent pyogenic infections, and progressive neurologic defects [1]. **Why other options are incorrect:** * **Ataxia Telangiectasia:** Caused by a mutation in the **ATM gene** (11q22), which is responsible for sensing DNA double-strand breaks [2]. It is a classic DNA repair defect. * **Bloom Syndrome:** Caused by a mutation in the **BLM gene** (encoding RecQ helicase), leading to genomic instability and a high frequency of sister chromatid exchanges. * **Seckel Syndrome:** A rare "primordial dwarfism" disorder caused by mutations in genes like **ATR**, which are critical for the cellular response to DNA replication stress. **High-Yield Clinical Pearls for NEET-PG:** * **Chediak-Higashi:** Look for **"Giant Granules"** in neutrophils on a peripheral smear [1]. * **DNA Repair Defects:** Often present with a predisposition to **malignancy** (especially lymphomas and leukemias) and **radiosensitivity** [2]. * **Wiskott-Aldrich Syndrome:** Another high-yield immunodeficiency; remember the triad: **T**hrombocytopenia (small platelets), **E**czema, and **R**ecurrent infections (**WATER**) [2]. * **Nijmegen Breakage Syndrome:** Another DNA repair defect often grouped with Ataxia Telangiectasia. **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. Diseases of the Immune System, pp. 250-251.

Question 196: Henoch-Schönlein purpura is characterized by all of the following except?

A. Thrombocytopenia (Correct Answer)
B. Glomerulonephritis
C. Arthralgia
D. Abdominal pain

Explanation: **Explanation:** **Henoch-Schönlein Purpura (HSP)**, now commonly referred to as **IgA Vasculitis**, is a small-vessel systemic vasculitis characterized by the deposition of IgA-dominant immune complexes. **Why Thrombocytopenia is the correct answer:** The hallmark of HSP is **non-thrombocytopenic purpura**. Unlike conditions like ITP or TTP where bruising occurs due to low platelet counts [2], the purpura in HSP is caused by **leukocytoclastic vasculitis** (inflammation of the blood vessel walls) [1]. Therefore, the platelet count in HSP is typically **normal or even elevated** (as an acute-phase reactant). Finding a low platelet count should prompt a clinician to look for an alternative diagnosis. **Analysis of Incorrect Options:** * **Glomerulonephritis:** Renal involvement occurs in about 40-50% of cases. It is histologically identical to IgA Nephropathy (Berger’s disease), presenting with hematuria and proteinuria. * **Arthralgia:** Migratory arthralgia or arthritis, typically affecting the large joints of the lower extremities (knees and ankles), is seen in approximately 75% of patients. * **Abdominal Pain:** Gastrointestinal involvement is common due to bowel wall edema and hemorrhage, often presenting as colicky pain, vomiting, or even intussusception (the most common GI complication). **High-Yield Clinical Pearls for NEET-PG:** * **Classic Tetrad:** Palpable purpura (without thrombocytopenia), arthritis/arthralgia, abdominal pain, and renal disease. * **Demographics:** Most common vasculitis in children; often follows an **Upper Respiratory Tract Infection (URTI)**. * **Pathology:** Immunofluorescence shows **mesangial IgA deposits** in the kidneys. * **Diagnosis:** Primarily clinical; skin biopsy shows leukocytoclastic vasculitis with IgA deposition [1]. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Cardiovascular Disease, pp. 279-280. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 947-948.

Question 197: Thymoma is associated with which of the following conditions?

A. Myasthenia gravis (Correct Answer)
B. Hypergammaglobulinemia
C. Systemic lupus erythematosus
D. Multiple sclerosis

Explanation: **Explanation:** **1. Why Myasthenia Gravis (MG) is correct:** Thymoma is a tumor derived from thymic epithelial cells. The thymus plays a critical role in T-cell maturation and self-tolerance. In patients with thymoma, there is a failure in the negative selection of T-cells, leading to the escape of autoreactive T-cells. These T-cells provide "help" to B-cells to produce antibodies against the **Acetylcholine Receptors (AChR)** at the neuromuscular junction [1]. * **High-yield correlation:** Approximately **10-15%** of patients with Myasthenia Gravis have a thymoma, while nearly **30-50%** of patients with a thymoma will develop Myasthenia Gravis. **2. Why other options are incorrect:** * **B. Hypergammaglobulinemia:** Thymoma is actually associated with **Hypogammaglobulinemia** (specifically **Good Syndrome**), characterized by thymoma, low B-cell counts, and hypogammaglobulinemia, leading to increased susceptibility to infections. * **C. Systemic Lupus Erythematosus (SLE):** While thymoma is associated with various autoimmune phenomena, SLE is not the primary or most characteristic association compared to MG. * **D. Multiple Sclerosis:** This is a demyelinating CNS disorder with no established clinical or pathological link to thymic tumors. **Clinical Pearls for NEET-PG:** * **Good Syndrome:** The triad of Thymoma + Hypogammaglobulinemia + Recurrent infections. * **Pure Red Cell Aplasia (PRCA):** Another classic paraneoplastic association of thymoma (seen in ~5% of cases). * **Morphology:** Look for "Hassall’s corpuscles" (though these are often absent in neoplastic tissue) [3] and a mixture of neoplastic epithelial cells and non-neoplastic lymphocytes [2]. * **Imaging:** A CT chest is the gold standard for identifying an anterior mediastinal mass in a patient presenting with muscle weakness. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1237-1238. [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. 634. [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, pp. 634-635.

Question 198: In Wegener's granulomatosis, cytoplasmic anti-neutrophilic antibodies are directed against which of the following?

A. Proteinase 1
B. Proteinase 2
C. Proteinase 3 (Correct Answer)
D. Proteinase 4

Explanation: **Explanation:** **Wegener’s Granulomatosis** (now commonly referred to as Granulomatosis with Polyangiitis or GPA) is a small-vessel vasculitis characterized by a triad of necrotizing granulomas of the respiratory tract, necrotizing vasculitis, and focal necrotizing glomerulonephritis [2]. **1. Why Proteinase 3 (PR3) is correct:** The hallmark of GPA is the presence of **c-ANCA** (cytoplasmic Anti-Neutrophil Cytoplasmic Antibodies). These antibodies show a diffuse granular cytoplasmic staining pattern on immunofluorescence. The primary target antigen for c-ANCA is **Proteinase 3 (PR3)**, a neutral serine protease found within the azurophilic granules of neutrophils [2]. The interaction between PR3-ANCA and neutrophils leads to excessive neutrophil activation, degranulation, and subsequent endothelial damage. **2. Why other options are incorrect:** * **Proteinase 1, 2, and 4:** These are not recognized as significant target antigens in the pathogenesis of ANCA-associated vasculitides. In medical pathology, the two clinically relevant antigens are **PR3** (associated with c-ANCA/GPA) and **Myeloperoxidase (MPO)** (associated with p-ANCA/Microscopic Polyangiitis and Churg-Strauss Syndrome). **High-Yield Clinical Pearls for NEET-PG:** * **c-ANCA (PR3-ANCA):** Highly specific for Wegener’s Granulomatosis (GPA) [2]. * **p-ANCA (MPO-ANCA):** Associated with Microscopic Polyangiitis (MPA) and Eosinophilic Granulomatosis with Polyangiitis (Churg-Strauss). * **Classic Triad of GPA:** Upper respiratory tract (sinusitis/saddle nose), Lower respiratory tract (hemoptysis/cavitation), and Kidneys (RPGN/Crescentic GN) [2]. * **Biopsy finding:** "Geographic necrosis" and poorly formed granulomas. Note that GPA is "Pauci-immune," meaning there is little to no antibody/complement deposition on immunofluorescence of the kidney [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 917-918. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 519-520.

Question 199: Acute Graft-versus-Host Disease (GVHD) is primarily caused by which type of lymphocyte?

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

Explanation: **Explanation:** **Acute Graft-versus-Host Disease (GVHD)** occurs when immunologically competent cells (the graft) are transplanted into an immunodeficient recipient (the host), and the graft cells recognize the host's tissues as foreign [1]. **Why T lymphocytes are the correct answer:** The pathogenesis of GVHD is driven by **donor T lymphocytes** (both CD4+ and CD8+) [2]. When these mature T cells are infused with the graft (commonly bone marrow or hematopoietic stem cells), they recognize the recipient's HLA antigens as foreign [2]. CD4+ cells release cytokines (cytokine storm), while CD8+ T cells directly cause cytotoxicity against host tissues. The primary targets are the **skin** (rash/dermatitis), **liver** (jaundice/bile duct destruction), and **gastrointestinal tract** (diarrhea/mucosal ulceration) [1]. **Why other options are incorrect:** * **B lymphocytes:** While B cells are involved in chronic GVHD (producing autoantibodies), they are not the primary initiators of the acute cellular attack. * **Macrophages:** These act as effector cells and antigen-presenting cells (APCs) that amplify the inflammatory response, but they do not initiate the specific immune recognition characteristic of GVHD. * **NK cells:** While they play a role in the innate immune response and "Graft-versus-Leukemia" effect, they are not the primary mediators of GVHD [2]. **High-Yield NEET-PG Pearls:** * **Prerequisites for GVHD (Billingham’s Criteria):** 1. Graft must contain immunocompetent cells; 2. Host must be immunocompromised; 3. Host must possess antigens foreign to the donor. * **Timeline:** Acute GVHD typically occurs within **100 days** of transplantation. * **Prophylaxis:** Depletion of donor T cells before transfusion can prevent GVHD, but it increases the risk of graft failure and recurrence of leukemia [2]. **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] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 244-245.

Question 200: Which of the following organs can cause an antigen reaction when exposed in self-blood?

A. Kidney
B. Liver
C. Eye lens (Correct Answer)
D. Platelets

Explanation: ### Explanation The correct answer is **C. Eye lens**. This question tests the concept of **Immunological Privilege** and **Sequestered Antigens**. **1. Why the Eye Lens is Correct:** During embryonic development, the immune system undergoes "tolerance induction," where T and B cells are taught to recognize "self" antigens [1]. However, certain tissues are anatomically isolated from the blood and lymphatic systems by physical barriers (e.g., the blood-brain barrier or the lens capsule). These are known as **sequestered antigens**. The **eye lens** proteins are sequestered from birth. Because the immune system has never "seen" these proteins, it does not recognize them as "self." If the lens capsule is ruptured due to trauma or surgery, these proteins leak into the systemic circulation. The immune system perceives them as foreign, triggering an inflammatory autoimmune response known as **Phacoantigenic Endophthalmitis**. **2. Why Other Options are Incorrect:** * **Kidney (A) and Liver (B):** These are highly vascular organs. Their antigens are constantly exposed to circulating lymphocytes during development and throughout life [1]. Therefore, the immune system recognizes them as "self," and they do not trigger an immediate antigenic reaction in a healthy individual. * **Platelets (D):** These are normal components of the blood. The immune system is fully tolerant of platelet surface antigens (unless a specific pathology like ITP occurs). **3. High-Yield Clinical Pearls for NEET-PG:** * **Examples of Sequestered Antigens:** Eye lens, Spermatozoa (testis), and Myelin Basic Protein (CNS) [2]. * **Sympathetic Ophthalmia:** A classic exam scenario where trauma to one eye (the exciting eye) leads to the release of sequestered antigens, causing an autoimmune attack on the *other* healthy eye (the sympathizing eye). * **Mechanism:** This represents a failure of **peripheral tolerance** due to the sudden release of antigens that were previously hidden from the immune system [1]. **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. 175-177. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 219-220.

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