Immunopathology — MCQs

Q: Molecular mimicry is an explanation for which of the following?

Autoimmune disorders. ### Explanation **Molecular Mimicry** is a key mechanism in the pathogenesis of **Autoimmune Disorders** [1]. It occurs when there is a structural similarity between the antigens of an infectious agent (bacteria or virus) and the host’s self-antigens. When the immune system mounts a response against the pathogen, the cross-reactive antibodies or T-cells mistakenly attack the host’s own tissues, leading to loss of self-tolerance and subsequent tissue damage [1]. **Why the other options are incorrect:** * **Immune Tolerance:** This is the state of unresponsiveness to an antigen. Molecular mimicry represents a *failure* of tolerance, specifically a breakdown in peripheral tolerance. * **Hypersensitivity:** While autoimmunity involves hypersensitivity reactions (Types II, III, or IV), "Hypersensitivity" is a broad category describing exaggerated immune responses to *exogenous* antigens (like pollen or drugs). Molecular mimicry specifically explains the *trigger* for attacking *endogenous* (self) antigens [1]. * **Immunosuppression:** This refers to a reduced activation or efficacy of the immune system (e.g., HIV or chemotherapy). Molecular mimicry involves an *overactive* and misdirected immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Example:** **Rheumatic Heart Disease.** Antibodies against the M-protein of *Streptococcus pyogenes* cross-react with cardiac myosin, leading to carditis. * **Guillain-Barré Syndrome (GBS):** Lipopolysaccharides of *Campylobacter jejuni* mimic gangliosides in peripheral nerves. * **Type 1 Diabetes:** Potential mimicry between Coxsackie B virus antigens and islet cell antigens (GAD65) [1]. * **Ankylosing Spondylitis:** Associated with *Klebsiella* species cross-reacting with HLA-B27. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 219-226.

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 1: Which of the following is NOT a disorder of phagocytosis?

A. Job's syndrome
B. Chediak-Higashi syndrome
C. Myeloperoxidase deficiency
D. Wiskott-Aldrich syndrome (Correct Answer)

Explanation: The correct answer is **Wiskott-Aldrich syndrome (WAS)** because it is primarily a **combined B-cell and T-cell immunodeficiency**, not a primary disorder of phagocytosis [1]. It is an X-linked recessive condition caused by a mutation in the *WASP* gene, which affects the actin cytoskeleton in hematopoietic cells [1]. This leads to the classic triad of **Thrombocytopenia** (with small platelets), **Eczema**, and **Recurrent infections**. **Analysis of other options (Disorders of Phagocytosis):** * **Job’s Syndrome (Hyper-IgE Syndrome):** A defect in JAK-STAT signaling (STAT3 mutation) leading to impaired neutrophil chemotaxis. It is characterized by "Cold" staphylococcal abscesses, retained primary teeth, and high IgE. * **Chediak-Higashi Syndrome:** A defect in vesicle fusion (LYST gene mutation) [2]. It results in impaired phagolysosome formation [2]. Key findings include giant cytoplasmic granules in neutrophils and partial albinism [2]. * **Myeloperoxidase (MPO) Deficiency:** The most common inherited defect of phagocytes. It involves a failure to produce Hypochlorous acid (HOCl), though most patients remain asymptomatic unless they have co-existing diabetes (predisposing to *Candida* infections). **NEET-PG High-Yield Pearls:** 1. **Phagocytosis Steps:** Remember the sequence: Chemotaxis → Opsonization → Ingestion → Killing (Oxidative burst). 2. **Nitroblue Tetrazolium (NBT) Test:** Used for Chronic Granulomatous Disease (CGD); it remains **negative** (colorless) in CGD due to NADPH oxidase deficiency. 3. **Wiskott-Aldrich Mnemonic:** **TIE** (Thrombocytopenia, Infections, Eczema). 4. **Small Platelets:** WAS is one of the few conditions where platelet size is decreased on a peripheral smear. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 250-251. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 245-246.

Question 2: Molecular mimicry is an explanation for which of the following?

A. Immune tolerance
B. Autoimmune disorders (Correct Answer)
C. Hypersensitivity
D. Immunosuppression

Explanation: ### Explanation **Molecular Mimicry** is a key mechanism in the pathogenesis of **Autoimmune Disorders** [1]. It occurs when there is a structural similarity between the antigens of an infectious agent (bacteria or virus) and the host’s self-antigens. When the immune system mounts a response against the pathogen, the cross-reactive antibodies or T-cells mistakenly attack the host’s own tissues, leading to loss of self-tolerance and subsequent tissue damage [1]. **Why the other options are incorrect:** * **Immune Tolerance:** This is the state of unresponsiveness to an antigen. Molecular mimicry represents a *failure* of tolerance, specifically a breakdown in peripheral tolerance. * **Hypersensitivity:** While autoimmunity involves hypersensitivity reactions (Types II, III, or IV), "Hypersensitivity" is a broad category describing exaggerated immune responses to *exogenous* antigens (like pollen or drugs). Molecular mimicry specifically explains the *trigger* for attacking *endogenous* (self) antigens [1]. * **Immunosuppression:** This refers to a reduced activation or efficacy of the immune system (e.g., HIV or chemotherapy). Molecular mimicry involves an *overactive* and misdirected immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Example:** **Rheumatic Heart Disease.** Antibodies against the M-protein of *Streptococcus pyogenes* cross-react with cardiac myosin, leading to carditis. * **Guillain-Barré Syndrome (GBS):** Lipopolysaccharides of *Campylobacter jejuni* mimic gangliosides in peripheral nerves. * **Type 1 Diabetes:** Potential mimicry between Coxsackie B virus antigens and islet cell antigens (GAD65) [1]. * **Ankylosing Spondylitis:** Associated with *Klebsiella* species cross-reacting with HLA-B27. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 219-226.

Question 3: Erythroblastosis fetalis is an example of which type of hypersensitivity reaction?

A. Type I
B. Type II (Correct Answer)
C. Type III
D. Type IV

Explanation: **Explanation:** **Erythroblastosis Fetalis (Hemolytic Disease of the Newborn)** is a classic example of **Type II Hypersensitivity**, also known as **Cytotoxic Hypersensitivity**. **Why Type II is correct:** Type II reactions are mediated by **IgG or IgM antibodies** directed against antigens present on the surface of specific cells or tissues [2]. In Erythroblastosis Fetalis, maternal IgG antibodies (produced after prior sensitization) cross the placenta and bind to Rh antigens on the fetal Red Blood Cells (RBCs) [1]. This leads to RBC destruction via two mechanisms: 1. **Opsonization and Phagocytosis** by splenic macrophages [2]. 2. **Complement-mediated lysis** [2]. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by **IgE** and mast cell degranulation (e.g., Anaphylaxis, Asthma) [3]. * **Type III (Immune-Complex):** Involves deposition of **antigen-antibody complexes** in tissues, leading to inflammation (e.g., SLE, Post-streptococcal glomerulonephritis). * **Type IV (Delayed):** Cell-mediated immunity involving **T-lymphocytes**, not antibodies (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Antibody-dependent cellular cytotoxicity (ADCC) is a key component of Type II reactions. * **Prevention:** Administer **Anti-D (RhoGAM)** to Rh-negative mothers at 28 weeks and within 72 hours of delivery to prevent sensitization [1]. * **Diagnosis:** The **Direct Coombs Test** is used to detect antibodies already bound to the baby's RBCs, while the **Indirect Coombs Test** checks the mother's serum for anti-Rh antibodies. * **Other Type II Examples:** Myasthenia Gravis, Graves' Disease, Goodpasture Syndrome, and Rheumatic Fever [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 469-470. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 214. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 212-213.

Question 4: A patient on treatment with penicillin developed pallor, but without shortness of breath, urticaria, or wheezing. Investigations revealed antibodies against penicillin in his blood. What is the most likely type of hypersensitivity reaction that occurred in this patient?

A. Type I hypersensitivity
B. Type II hypersensitivity (Correct Answer)
C. Type III hypersensitivity
D. Type IV hypersensitivity

Explanation: ### Explanation **Why Type II Hypersensitivity is Correct:** This scenario describes **Drug-Induced Hemolytic Anemia**, a classic example of Type II (Antibody-mediated/Cytotoxic) hypersensitivity. In this mechanism, penicillin acts as a **hapten**; it binds to the surface of red blood cells (RBCs) [1]. The immune system then produces IgG or IgM antibodies against this drug-protein complex. These antibodies bind to the RBCs, leading to their destruction via the complement system or phagocytosis by splenic macrophages (opsonization) [2]. The clinical presentation of **pallor** without systemic anaphylactic symptoms (like wheezing or urticaria) points specifically to targeted cell destruction rather than a systemic allergic response [1]. **Why the Other Options are Incorrect:** * **Type I (Immediate):** Mediated by IgE and mast cell degranulation. It would typically present with urticaria, angioedema, wheezing, or anaphylaxis. The question specifically excludes these symptoms. * **Type III (Immune-Complex):** Involves the deposition of antigen-antibody complexes in tissues (e.g., Serum Sickness) [3]. It usually presents with fever, joint pain, and rashes, rather than isolated pallor/anemia. * **Type IV (Delayed):** T-cell mediated and does not involve antibodies. Examples include contact dermatitis or the Mantoux test. **High-Yield Clinical Pearls for NEET-PG:** * **Coombs Test:** The Direct Antiglobulin Test (DAT) is the gold standard for diagnosing Type II drug-induced hemolysis (detects IgG/complement on the RBC surface) [1]. * **Common Triggers:** Besides Penicillin, other drugs causing Type II reactions include **Quinidine** (thrombocytopenia) and **Methyldopa** (autoimmune hemolytic anemia) [1]. * **Mnemonic for Hypersensitivity (ACID):** * **A** - **A**naphylactic (Type I) * **C** - **C**ytotoxic (Type II) * **I** - **I**mmune Complex (Type III) * **D** - **D**elayed-type (Type IV) **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 651-652. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 214. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 214-215.

Question 5: Asthma is classified as which type of hypersensitivity reaction?

A. Type I hypersensitivity (Correct Answer)
B. Type II hypersensitivity
C. Type III hypersensitivity
D. Type IV hypersensitivity

Explanation: **Explanation:** **Type I Hypersensitivity (Immediate Hypersensitivity)** is the correct answer. Bronchial asthma (specifically atopic asthma) is mediated by an **IgE-dependent mechanism** [2]. Upon exposure to an allergen, Th2 cells stimulate B-cells to produce IgE, which binds to the surface of **mast cells** [3]. Subsequent exposure leads to antigen cross-linking of IgE, causing mast cell degranulation and the release of mediators like histamine, leukotrienes (C4, D4, E4), and prostaglandins [1]. This results in smooth muscle contraction (bronchospasm), mucosal edema, and mucus hypersecretion [5]. **Why other options are incorrect:** * **Type II (Antibody-mediated):** Involves IgG or IgM antibodies binding to fixed antigens on cell surfaces or tissues (e.g., Goodpasture syndrome, Myasthenia gravis) [2]. * **Type III (Immune complex-mediated):** Caused by the deposition of antigen-antibody complexes in tissues, leading to complement activation (e.g., SLE, Post-streptococcal glomerulonephritis) [2]. * **Type IV (Cell-mediated/Delayed):** Mediated by T-lymphocytes rather than antibodies. It typically takes 48–72 hours to manifest (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Key Cytokines:** Th2 cells produce **IL-4** (stimulates IgE switch), **IL-5** (activates eosinophils), and **IL-13** (stimulates mucus secretion) [3]. * **Curschmann Spirals & Charcot-Leyden Crystals:** Classic microscopic findings in the sputum of asthmatic patients. * **Late-phase reaction:** Occurs 2–24 hours after exposure, primarily driven by **eosinophils** recruited by IL-5 [4]. * **Drug-induced Asthma:** Aspirin-induced asthma is a non-immune variant involving the cyclooxygenase pathway, not Type I hypersensitivity. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 210-211. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 208-210. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 688-689. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 210. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 211-212.

Question 6: All are true regarding Severe Combined Immunodeficiency (SCID) except:

A. Due to deficiency of adenosine deaminase
B. Toxicity of leucocytes due to accumulation of cyclic adenosine monophosphate (cAMP) (Correct Answer)
C. Both cellular and humoral immunity are affected
D. Bone marrow transplantation is a treatment option

Explanation: **Explanation:** Severe Combined Immunodeficiency (SCID) is a group of rare disorders characterized by the profound deficiency of both T-cell and B-cell functions [2]. **Why Option B is the correct answer (The False Statement):** In the autosomal recessive form of SCID caused by **Adenosine Deaminase (ADA) deficiency**, the lack of the enzyme leads to the accumulation of **deoxyadenosine** and its derivatives, specifically **deoxy-ATP (dATP)** [2]. High levels of dATP are toxic to lymphocytes because they inhibit ribonucleotide reductase, thereby preventing DNA synthesis and leading to cell death [2]. The toxicity is **not** due to the accumulation of cyclic adenosine monophosphate (cAMP). **Analysis of other options:** * **Option A:** ADA deficiency is the second most common cause of SCID (approx. 20% of cases) and the most common autosomal recessive form [2]. * **Option C:** By definition, SCID involves a "combined" defect. Patients have profound lymphopenia, affecting both **cellular immunity** (T-cells) and **humoral immunity** (B-cells/antibodies) [3]. * **Option D:** **HSC (Bone Marrow) Transplantation** is the definitive treatment of choice [1]. Without it, SCID is usually fatal within the first year of life [1]. ADA deficiency is also notable for being the first disease treated with human gene therapy [1]. **NEET-PG High-Yield Pearls:** * **Most common cause:** X-linked SCID due to a mutation in the **$\gamma$c chain** of cytokine receptors (IL-2RG) [2]. * **Clinical presentation:** Recurrent severe infections (diarrhea, pneumonia, thrush), failure to thrive, and an **absent thymic shadow** on X-ray. * **Laboratory findings:** Very low absolute lymphocyte count and "bubble boy" pathology (hypoplastic thymus and peripheral lymphoid tissue). **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. Diseases of the Immune System, pp. 247-248. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 246-247.

Question 7: MHC is important in the pathogenesis of which of the following conditions?

A. Idiopathic disease
B. Iatrogenic diseases
C. Auto-immune diseases (Correct Answer)
D. Tumours

Explanation: **Explanation:** **Major Histocompatibility Complex (MHC)**, known as Human Leukocyte Antigen (HLA) in humans, plays a pivotal role in the pathogenesis of **Auto-immune diseases** [1]. The fundamental function of MHC molecules is to present peptide antigens to T-cells. Autoimmunity occurs when there is a breakdown in self-tolerance, often linked to specific MHC alleles that efficiently present "self-antigens" to autoreactive T-cells or fail to eliminate them during thymic selection [3]. * **Why Option C is correct:** Many autoimmune diseases show a strong genetic association with specific HLA types [1]. For example, **HLA-B27** is strongly linked to Ankylosing Spondylitis, and **HLA-DR3/DR4** are associated with Type 1 Diabetes Mellitus and Rheumatoid Arthritis [2]. * **Why Options A & B are incorrect:** **Idiopathic diseases** have unknown causes, and while genetics may play a role, they are not defined by MHC interactions. **Iatrogenic diseases** are induced by medical treatment or diagnostic procedures (e.g., drug side effects), which are external factors independent of the MHC system. * **Why Option D is incorrect:** While the immune system uses MHC to detect **Tumours** (immunosurveillance), the *pathogenesis* (origin) of cancer is primarily driven by genetic mutations in proto-oncogenes and tumor suppressor genes, not the MHC system itself. **High-Yield Clinical Pearls for NEET-PG:** * **HLA-B27:** Ankylosing spondylitis, Reiter’s syndrome, Acute anterior uveitis [2]. * **HLA-DR4:** Rheumatoid arthritis. * **HLA-DQ2/DQ8:** Celiac disease. * **HLA-DR3:** SLE, Graves' disease, Type 1 Diabetes [2]. * **MHC Class I** (A, B, C) presents to **CD8+** T-cells; **MHC Class II** (DP, DQ, DR) presents to **CD4+** T-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. 177-178. [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. 49-50. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 222-223.

Question 8: Which immunoglobulin is predominantly found in breast milk and other secretions?

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

Explanation: **Explanation:** **IgA (Immunoglobulin A)** is the correct answer because it is the primary antibody class found in **secretions** (mucosal immunity). In breast milk, specifically colostrum, secretory IgA provides critical passive immunity to the neonate, protecting the infant's gastrointestinal tract against pathogens. It exists as a monomer in the blood but as a **dimer** in secretions, held together by a **J-chain** and a **secretory component** that protects it from enzymatic degradation in harsh environments like the gut. **Why other options are incorrect:** * **IgG:** This is the most abundant immunoglobulin in the serum and the only one that **crosses the placenta** to provide fetal immunity. It is not the primary secretory antibody. * **IgE:** Primarily involved in **Type I hypersensitivity** reactions (allergies) and host defense against helminthic (parasitic) infections [1]. It is found in very low concentrations in serum [1]. * **IgD:** Found mainly on the surface of B-cells where it functions as an antigen receptor; its secreted role is minimal and not significant in mucosal secretions [1]. **High-Yield NEET-PG Pearls:** * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are often asymptomatic but may present with recurrent sinopulmonary or GI infections [2]. * **Milk Immunity:** While IgG crosses the placenta (prenatal), **IgA** is the star of breast milk (postnatal). * **Structure:** IgA is the only immunoglobulin that contains a "secretory component" derived from epithelial cells. * **Agglutination:** IgA is effective at neutralizing toxins and preventing bacterial attachment to mucosal surfaces (immune exclusion). **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. 155-156. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 249-250.

Question 9: Anti-Ro antibody is associated with which of the following conditions?

A. Systemic Lupus Erythematosus (SLE)
B. Scleroderma
C. Mixed Connective Tissue Disease (MCTD)
D. Neonatal Lupus (Correct Answer)

Explanation: **Explanation:** **Anti-Ro (SS-A) antibodies** are autoantibodies directed against ribonucleoprotein complexes [3]. While they are frequently found in Sjögren’s syndrome and Systemic Lupus Erythematosus (SLE), they are the **pathognomonic marker for Neonatal Lupus**. [1] 1. **Why Neonatal Lupus is correct:** Neonatal lupus is caused by the transplacental passage of maternal IgG antibodies (specifically Anti-Ro/SS-A and Anti-La/SS-B). These antibodies can cause cutaneous lesions and, most critically, permanent **congenital heart block** by inducing inflammation and fibrosis in the fetal AV node. 2. **Why other options are incorrect:** * **SLE:** While Anti-Ro is present in 30-50% of SLE cases (especially in subacute cutaneous lupus), it is not the most specific or defining association among the choices provided [2]. * **Scleroderma:** This is primarily associated with **Anti-Scl-70** (diffuse) or **Anti-centromere** antibodies (limited/CREST) [1]. * **MCTD:** This condition is characterized by high titers of **Anti-U1 RNP** antibodies. **High-Yield Clinical Pearls for NEET-PG:** * **Anti-Ro (SS-A) and Anti-La (SS-B):** Most commonly associated with **Sjögren’s syndrome** (Ro is more common than La) [1]. * **Neonatal Lupus:** If a mother has Anti-Ro antibodies, there is a risk of the infant developing a third-degree heart block, which is irreversible. * **Subacute Cutaneous Lupus Erythematosus (SCLE):** Strongly associated with Anti-Ro (approx. 70-90% of cases). * **ANA-negative SLE:** Patients who are ANA-negative but have clinical SLE often test positive for Anti-Ro. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 236. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 226. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 228.

Question 10: All of the following are disorders of phagocytosis except?

A. Chronic granulomatous disease
B. Myeloperoxidase deficiency
C. Chediak-Higashi Syndrome
D. Nezelof Syndrome (Correct Answer)

Explanation: ### Explanation The correct answer is **Nezelof Syndrome**. #### 1. Why Nezelof Syndrome is the correct answer: Nezelof Syndrome is a **T-cell immunodeficiency** characterized by thymic hypoplasia, leading to impaired cell-mediated immunity [1]. Unlike the other options, it is a primary cellular immunodeficiency disorder, not a defect in phagocytic function [1]. While B-cell function may be affected (variable antibody levels), the primary pathology lies in the lymphoid lineage rather than the myeloid/phagocytic lineage [1]. #### 2. Analysis of Incorrect Options (Phagocytic Disorders): * **Chronic Granulomatous Disease (CGD):** A defect in **NADPH oxidase**, preventing the respiratory burst. Phagocytes can ingest bacteria but cannot produce superoxide radicals to kill them (specifically catalase-positive organisms). * **Myeloperoxidase (MPO) Deficiency:** The most common inherited phagocytic defect. It involves a failure to convert $H_2O_2$ to $HOCl$ (hypochlorous acid). While most patients are asymptomatic, it represents a clear defect in the **microbicidal (killing) phase** of phagocytosis. * **Chediak-Higashi Syndrome:** A defect in the **LYST gene** (lysosomal trafficking regulator), leading to impaired phagosome-lysosome fusion [2]. It is characterized by giant cytoplasmic granules in neutrophils [2]. #### 3. High-Yield Clinical Pearls for NEET-PG: * **Screening Test for CGD:** Nitroblue Tetrazolium (NBT) dye reduction test (Negative/No blue color in CGD) or the more modern Dihydrorhodamine (DHR) flow cytometry. * **Chediak-Higashi Triad:** Partial albinism, recurrent pyogenic infections, and peripheral neuropathy [2]. * **MPO Deficiency:** Characterized by a positive NBT test (since NADPH oxidase is intact) but impaired killing of *Candida albicans*. * **Nezelof Syndrome vs. DiGeorge:** Both involve thymic hypoplasia, but Nezelof Syndrome lacks the characteristic facies and hypocalcemia (parathyroid defects) seen in DiGeorge. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 247-248. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 245-246.

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Cells and Tissues of the Immune System

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Innate Immunity

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Adaptive Immunity

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Hypersensitivity Reactions

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Autoimmune Diseases

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Immunodeficiency Disorders

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Transplantation Immunopathology

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Immune Response to Infections

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Immunologic Laboratory Techniques

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Tumor Immunology

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