Immunopathology — MCQs

A 52-year-old woman with a history of systemic hypertension and chronic renal failure undergoes kidney transplantation, but the graft fails to produce urine. A renal biopsy is diagnosed as "hyperacute transplant rejection." Graft rejection in this patient is caused primarily by which of the following mediators of immunity and inflammation?

Q28Easy

TH1 is involved in which type of hypersensitivity?

Q29Medium

During heterosexual intercourse, seminal fluid containing HIV contacts vaginal squamous mucosa. Cells capture virions and transport the virus via lymphatics to regional lymph nodes. Within the germinal centers of these lymph nodes, the virions infect CD4+ lymphocytes and proliferate, causing CD4+ cell lysis with release of more virions, which are taken up on the surface of cells having Fc receptors, allowing continued infection by HIV of more CD4+ cells passing through the nodes. Which of the following types of cells is most likely to capture HIV on its surface via Fc receptors?

Q30Medium

Humoral immunodeficiency is suspected in a patient and he is under investigation. Which of the following infections would not be consistent with the diagnosis?

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 21: In myasthenia gravis, what deposits are found at the neuromuscular junctions?

A. Acetylcholine
B. Acetylcholine Receptor Antigens
C. Acetylcholinesterase
D. Immunoglobulin (Correct Answer)

Explanation: **Explanation:** **Myasthenia Gravis (MG)** is a classic example of a **Type II Hypersensitivity reaction** [1]. The pathogenesis involves the production of autoantibodies (predominantly IgG) directed against the postsynaptic **Nicotinic Acetylcholine Receptors (AChR)** at the neuromuscular junction (NMJ) [2]. 1. **Why Immunoglobulin is correct:** In MG, **IgG antibodies** bind to the AChR [2]. This leads to three primary mechanisms of dysfunction: * **Complement-mediated damage:** Activation of the classical complement pathway leads to the formation of the Membrane Attack Complex (MAC), causing focal destruction of the postsynaptic membrane. * **Receptor Internalization:** Cross-linking of receptors by antibodies leads to accelerated endocytosis and degradation. * **Direct Blockade:** Physical blocking of the binding site for acetylcholine [1]. Immunofluorescence studies of the NMJ in MG patients consistently show deposits of **IgG and Complement (C3, C9)**. 2. **Why other options are incorrect:** * **Acetylcholine (ACh):** This is the neurotransmitter itself. In MG, the amount of ACh released is normal, but it cannot bind effectively due to receptor loss. * **AChR Antigens:** These are the *targets* of the immune response, not the "deposits" that characterize the pathology. * **Acetylcholinesterase:** This is the enzyme that degrades ACh. While drugs (Pyridostigmine) inhibit this enzyme to treat MG, it is not a pathological deposit found at the NMJ. **High-Yield Clinical Pearls for NEET-PG:** * **Associated Pathology:** 75% of patients have **Thymic abnormalities** (65% Thymic Hyperplasia; 10% Thymoma). * **Antibody Profile:** 85% are AChR-Ab positive [2]. In seronegative cases, look for **MuSK (Muscle-Specific Kinase) antibodies**. * **Clinical Hallmark:** Fatigable muscle weakness (worse at the end of the day) and ptosis. * **Morphology:** On electron microscopy, there is **simplification of the postsynaptic folds** and a widened synaptic cleft. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 213-214. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1237-1238.

Question 22: Wiskott Aldrich syndrome is associated with all of the following except:

A. Thrombocytopenia
B. Recurrent infections
C. Low levels of IgG (Correct Answer)
D. Deficient cell mediated immunity

Explanation: **Wiskott-Aldrich Syndrome (WAS)** is an X-linked recessive immunodeficiency caused by a mutation in the **WASp gene**, which encodes a protein involved in actin cytoskeleton remodeling in hematopoietic cells [1]. This defect leads to impaired cell signaling and immunological synapse formation. ### **Explanation of the Correct Answer** * **Option C (Low levels of IgG):** In WAS, the serum levels of **IgG are typically normal**. The characteristic immunoglobulin profile shows **low IgM**, **elevated IgA**, and **elevated IgE** [1]. Because IgG levels are generally preserved, this option is the "except" and the correct answer. ### **Analysis of Incorrect Options** * **Option A (Thrombocytopenia):** This is a hallmark of WAS. Patients present with microthrombocytopenia (small, few platelets) due to both decreased production and increased splenic clearance. * **Option B (Recurrent infections):** Due to combined B-cell and T-cell dysfunction, patients are highly susceptible to encapsulated bacteria (e.g., *S. pneumoniae*), viruses, and *Pneumocystis jirovecii* [2]. * **Option D (Deficient cell-mediated immunity):** WASp is critical for T-cell migration and activation [1]. Over time, T-cell numbers and function decline, leading to progressive cellular immunodeficiency. ### **NEET-PG High-Yield Pearls** * **Classic Triad:** Thrombocytopenia (bleeding/petechiae), Eczema, and Recurrent infections (**"TIE"** mnemonic). * **Immunology Profile:** ↓ IgM, ↑ IgA, ↑ IgE, Normal IgG [1]. * **Platelet Morphology:** It is the only condition characterized by **small platelets** (low Mean Platelet Volume). * **Complications:** High risk of autoimmune hemolytic anemia and B-cell lymphomas [1]. * **Definitive Treatment:** Hematopoietic stem cell transplant [1]. **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] 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.

Question 23: Thymic hyperplasia is seen in which of the following conditions?

A. Thymoma
B. Thymic lymphoma
C. Myasthenia gravis (Correct Answer)
D. Scleroderma

Explanation: ### **Explanation** **1. Why Myasthenia Gravis (MG) is Correct:** Thymic hyperplasia (specifically **follicular hyperplasia**) is characterized by the presence of B-cell germinal centers within the thymic medulla [1]. This is a hallmark finding in **65–75% of patients with Myasthenia Gravis** [3]. * **Pathophysiology:** The thymus plays a central role in the pathogenesis of MG. It contains "myoid cells" (muscle-like cells) that express acetylcholine receptors (AChR) [1]. In hyperplasia, the thymus acts as a site of autosensitization, where B-cells produce pathogenic anti-AChR antibodies, leading to the classic neuromuscular junction defect [3]. **2. Analysis of Incorrect Options:** * **A. Thymoma:** While 10–15% of MG patients have a thymoma (a true neoplasm of thymic epithelial cells), thymoma and hyperplasia are distinct pathological entities [2]. Hyperplasia refers to lymphoid follicle formation, not a neoplastic growth. * **B. Thymic Lymphoma:** This is a malignancy of the lymphoid tissue (e.g., T-lymphoblastic lymphoma). It involves the clonal expansion of malignant cells rather than the reactive follicular hyperplasia seen in autoimmune conditions [2]. * **C. Scleroderma:** This is a systemic connective tissue disorder characterized by fibrosis. It is not classically associated with thymic hyperplasia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Thymic Hyperplasia vs. Thymoma:** Hyperplasia is more common in **younger females**, whereas Thymoma is more common in **older adults (40–60 years)**. * **Treatment Link:** Thymectomy often leads to clinical improvement or remission in MG patients with thymic hyperplasia. * **Other Associations:** Besides MG, thymic hyperplasia can occasionally be seen in other autoimmune diseases like SLE, Graves' disease, and Rheumatoid Arthritis [4]. * **Microscopic Hallmark:** Look for **"Germinal Centers"** in the medulla to diagnose follicular hyperplasia [1][2]. **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, p. 634. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 571-572. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1237-1238. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1092-1093.

Question 24: All of the following are true about MHC class I molecules except?

A. Present on antigen-presenting cells
B. Activate cytotoxic T cells to kill virus-infected cells
C. Present on nucleated cells
D. A first-line defense mechanism (Correct Answer)

Explanation: **Explanation:** The Major Histocompatibility Complex (MHC) Class I molecules are fundamental components of the **adaptive immune system**, not the innate (first-line) defense mechanism. **Why Option D is the Correct Answer (The False Statement):** First-line defense mechanisms include physical barriers (skin, mucus), chemical barriers (gastric acid), and the innate immune system (neutrophils, macrophages, NK cells). MHC molecules are part of the **adaptive immune response**, specifically involved in antigen presentation to T-lymphocytes, which takes time to develop and is highly specific [4]. **Analysis of Other Options:** * **Option A (True):** MHC Class I molecules are present on **all nucleated cells** [1], which includes professional Antigen-Presenting Cells (APCs) like dendritic cells, macrophages, and B-cells [2]. * **Option B (True):** MHC Class I molecules present endogenous antigens (e.g., viral proteins synthesized within a cell) to **CD8+ Cytotoxic T cells** [3]. This triggers the destruction of the infected cell. * **Option C (True):** MHC Class I is expressed on virtually **all nucleated cells** and platelets [2]. Notably, they are **absent on mature Red Blood Cells (RBCs)** because RBCs lack a nucleus. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** MHC Class I consists of an **alpha (α) heavy chain** (encoded on Chromosome 6) and a **β2-microglobulin** light chain (encoded on Chromosome 15). * **Binding Site:** The peptide-binding cleft is formed by the **α1 and α2** domains [2]. * **MHC Restriction:** CD8+ T cells only recognize antigens when presented on MHC I ("Rule of 8": 8 × 1 = 8). * **Human Analogue:** In humans, MHC is referred to as **HLA (Human Leukocyte Antigen)**. HLA-A, B, and C correspond to MHC Class I [2]. **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. 156-157. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 240. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 207-208.

Question 25: Which of the following conditions is NOT associated with ANCA?

A. Microscopic polyangitis
B. Churg-Strauss syndrome
C. Wegener's granulomatosis
D. Henoch-Schonlein purpura (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Henoch-Schonlein purpura (HSP)**. **Why HSP is the correct answer:** Henoch-Schönlein Purpura (now termed IgA Vasculitis) is a **small-vessel vasculitis** characterized by the deposition of **IgA-dominant immune complexes** in vessel walls. It is a Type III hypersensitivity reaction. Unlike the other options, it is **not** associated with Anti-Neutrophil Cytoplasmic Antibodies (ANCA) [1]. Diagnosis is typically based on the clinical tetrad of palpable purpura, arthralgia, abdominal pain, and renal involvement (IgA nephropathy). **Why the other options are incorrect:** Options A, B, and C represent the **ANCA-associated vasculitides (AAV)**, which are characterized by "pauci-immune" inflammation (lack of significant antibody/complement deposition) [3]: * **Microscopic Polyangiitis (MPA):** Strongly associated with **p-ANCA** (anti-MPO) [3]. It involves small vessels and often causes necrotizing glomerulonephritis and pulmonary capillaritis. * **Churg-Strauss Syndrome (EGPA):** Associated with **p-ANCA** (in ~40-50% of cases). Key features include asthma, eosinophilia, and granulomatous inflammation. * **Wegener’s Granulomatosis (GPA):** Strongly associated with **c-ANCA** (anti-PR3) [2]. It is characterized by the triad of upper respiratory tract involvement, lower respiratory tract involvement, and renal disease. **NEET-PG High-Yield Pearls:** * **c-ANCA (Cytoplasmic):** Targets Proteinase-3 (**PR3**); most specific for Wegener’s (GPA) [2]. * **p-ANCA (Perinuclear):** Targets Myeloperoxidase (**MPO**); associated with MPA and Churg-Strauss [3]. * **Pauci-immune:** This term is a hallmark of ANCA-associated vasculitis on immunofluorescence (minimal Ig/complement staining), distinguishing it from HSP or SLE [3]. * **HSP** is the most common vasculitis in children and often follows an upper respiratory tract infection. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 514-515. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 519-520. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 518-519.

Question 26: Which of the following statements regarding C-reactive protein is false?

A. It is increased in pneumococcal infection.
B. It is an acute phase substance.
C. The gene for the protein is on Chromosome 14. (Correct Answer)
D. It can be raised after surgery.

Explanation: **Explanation:** C-reactive protein (CRP) is a classic **Acute Phase Reactant (APR)** synthesized by the liver in response to pro-inflammatory cytokines, primarily **Interleukin-6 (IL-6)** [1][2]. **Why Option C is the correct (False) statement:** The gene encoding C-reactive protein (*CRP* gene) is located on **Chromosome 1**, not Chromosome 14. This is a high-yield genetic fact often tested in pathology. Chromosome 14 is notable for housing the genes for the Immunoglobulin Heavy chain (IgH) and Alpha-1 Antitrypsin. **Analysis of other options:** * **Option A (True):** CRP was originally discovered in the serum of patients with pneumonia; it reacts with the **C-polysaccharide** of the cell wall of *Streptococcus pneumoniae*, hence the name "C-reactive" protein. * **Option B (True):** It is a "positive" acute phase substance. Its levels rise rapidly (within 6–12 hours) and significantly (up to 1000-fold) during acute inflammation, tissue injury, or infection [2]. * **Option D (True):** Since surgery involves significant tissue trauma, it triggers a systemic inflammatory response, leading to a predictable rise in CRP levels post-operatively. **High-Yield Clinical Pearls for NEET-PG:** * **Function:** CRP acts as an **opsonin**, binding to phosphocholine on microbes and damaged cells to facilitate phagocytosis and activate the classical complement pathway (C1q) [1]. * **Half-life:** It has a constant half-life of about **19 hours**, making its serum concentration solely dependent on the rate of production (reflecting the severity of inflammation). * **hs-CRP:** High-sensitivity CRP is used as a biomarker for **cardiovascular risk stratification**, reflecting low-grade chronic inflammation in atherosclerosis [1]. * **ESR vs. CRP:** CRP is a more sensitive and faster indicator of acute inflammation than the Erythrocyte Sedimentation Rate (ESR). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 501-502. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, p. 111.

Question 27: A 52-year-old woman with a history of systemic hypertension and chronic renal failure undergoes kidney transplantation, but the graft fails to produce urine. A renal biopsy is diagnosed as "hyperacute transplant rejection." Graft rejection in this patient is caused primarily by which of the following mediators of immunity and inflammation?

A. Cytotoxic T lymphocytes
B. Helper T lymphocytes
C. Mononuclear phagocytes
D. Preformed antibodies (Correct Answer)

Explanation: ### Explanation **Hyperacute rejection** is a Type II hypersensitivity reaction that occurs within minutes to hours after transplantation. [1] **1. Why the Correct Answer is Right:** The primary mechanism is the presence of **preformed antibodies** (IgG or IgM) in the recipient's serum that are specific for antigens on the donor vascular endothelium (usually ABO blood group antigens or HLA Class I). [2] * **Mechanism:** Once the graft is vascularized, these antibodies bind to the donor endothelium, activating the **classical complement pathway**. [1] * **Consequence:** This leads to endothelial injury, fibrin-platelet thrombi formation, and neutrophilic infiltration. [3] The result is widespread **thrombotic microangiopathy**, leading to graft ischemia, infarction, and a "cyanotic" non-functional kidney (no urine production). [1] **2. Why the Other Options are Incorrect:** * **A & B (Cytotoxic and Helper T cells):** These are the primary mediators of **Acute Cellular Rejection**. [3] This typically occurs days to weeks after transplant (not immediately) because it requires time for the recipient's T cells to be sensitized to the donor's alloantigens. [4] * **C (Mononuclear phagocytes):** While macrophages play a role in chronic rejection and late-stage acute rejection through cytokine release and tissue remodeling, they are not the primary initiators of the rapid, fulminant vascular destruction seen in hyperacute rejection. **3. NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Occurs "on the operating table"; the graft turns blue/mottled and fails to produce urine. [1] * **Morphology:** Fibrinoid necrosis of arterial walls and thrombotic occlusion of capillaries. [3] * **Prevention:** Mandatory **Cross-matching** (testing recipient serum against donor lymphocytes) has made hyperacute rejection rare in modern practice. * **Timeline Recap:** * **Hyperacute:** Minutes/Hours (Preformed Antibodies). * **Acute:** Days/Weeks (T-cells or Antibodies). * **Chronic:** Months/Years (Intimal thickening/fibrosis). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 241-242. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 213-214. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 242. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 240-241.

Question 28: TH1 is involved in which type of hypersensitivity?

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

Explanation: ### Explanation **Correct Option: D (Type 4 Hypersensitivity)** **Mechanism:** Type 4 hypersensitivity is also known as **Delayed-Type Hypersensitivity (DTH)** [1], [2]. Unlike Types 1, 2, and 3, it is **cell-mediated** rather than antibody-mediated. The process is primarily driven by **CD4+ T-helper 1 (TH1) cells** [2]. Upon exposure to an antigen, TH1 cells secrete cytokines, most notably **Interferon-gamma (IFN-̳)** [1], [5]. IFN-̳ is the potent activator of macrophages, leading to tissue injury, granuloma formation, and inflammation [5]. Another subset, TH17 cells, also contributes by recruiting neutrophils [2]. **Why other options are incorrect:** * **Type 1 (Immediate):** Mediated by **IgE antibodies** and **TH2 cells** (which secrete IL-4 and IL-5). It involves mast cell degranulation and histamine release (e.g., Anaphylaxis, Asthma). * **Type 2 (Antibody-mediated):** Mediated by **IgG or IgM** antibodies directed against target antigens on specific cell surfaces or tissues (e.g., Myasthenia Gravis, Rheumatic Fever). * **Type 3 (Immune-complex):** Mediated by the deposition of **antigen-antibody complexes** in tissues, leading to complement activation and neutrophil recruitment (e.g., SLE, Post-streptococcal glomerulonephritis). **High-Yield NEET-PG Pearls:** 1. **Mnemonic for Hypersensitivity (ACID):** **A**naphylactic (Type 1), **C**ytotoxic (Type 2), **I**mmune-Complex (Type 3), **D**elayed (Type 4). 2. **Classic Examples of Type 4:** Mantoux (Tuberculin) test [4], Contact dermatitis (Poison ivy/Nickel) [3], and Granulomatous diseases (Tuberculosis, Sarcoidosis). 3. **Key Cytokine:** IFN-̳ is the hallmark cytokine of the TH1 response and is essential for macrophage activation [5]. 4. **Time Frame:** Type 4 reactions typically peak at **48–72 hours** post-exposure [1], [4]. **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] 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. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 218. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 206.

Question 29: During heterosexual intercourse, seminal fluid containing HIV contacts vaginal squamous mucosa. Cells capture virions and transport the virus via lymphatics to regional lymph nodes. Within the germinal centers of these lymph nodes, the virions infect CD4+ lymphocytes and proliferate, causing CD4+ cell lysis with release of more virions, which are taken up on the surface of cells having Fc receptors, allowing continued infection by HIV of more CD4+ cells passing through the nodes. Which of the following types of cells is most likely to capture HIV on its surface via Fc receptors?

A. B lymphocyte
B. CD8+ cytotoxic lymphocyte
C. Follicular dendritic cell (Correct Answer)
D. Natural Killer cell

Explanation: **Explanation:** The correct answer is **Follicular Dendritic Cell (FDC)**. **1. Why Follicular Dendritic Cells are correct:** In the early and chronic phases of HIV infection, the germinal centers of regional lymph nodes serve as major reservoirs for the virus. FDCs are specialized APCs located in the germinal centers. Unlike typical dendritic cells, FDCs possess high-affinity **Fc receptors** (for IgG) and **Complement receptors** (C3b/C3d). They trap HIV virions that have been opsonized by antibodies or complement. These trapped virions remain on the FDC surface for long periods without being internalized, acting as a "viral warehouse" [1]. As naive CD4+ T cells migrate through the lymph node to interact with B cells, they come into contact with these surface-bound virions, leading to persistent infection and progressive CD4+ depletion. **2. Why other options are incorrect:** * **B Lymphocytes:** While B cells reside in germinal centers and are involved in the immune response, they do not act as the primary reservoir for trapping extracellular HIV via Fc receptors in this manner. * **CD8+ Cytotoxic Lymphocytes:** These cells are responsible for killing virus-infected cells [1]. They do not possess the specific Fc-receptor mechanism required to capture and "store" HIV virions for presentation to CD4+ cells. * **Natural Killer (NK) Cells:** Although NK cells have Fc receptors (CD16) used for Antibody-Dependent Cellular Cytotoxicity (ADCC), they do not reside in the germinal center follicles to trap and transmit HIV to CD4+ cells. **Clinical Pearls for NEET-PG:** * **Initial Capture:** HIV is first captured in the mucosa by **Langerhans cells** (using the lectin receptor *Langerin*). * **The Reservoir:** During the "clinical latency" phase, the virus is not truly latent; it is actively replicating within the **lymph node follicles**, trapped by FDCs [2]. * **Coreceptors:** HIV uses **gp120** to bind to CD4. It requires co-receptors: **CCR5** (Macrophage-tropic, early infection) or **CXCR4** (T-cell-tropic, late infection). **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. 555-556. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 258-259.

Question 30: Humoral immunodeficiency is suspected in a patient and he is under investigation. Which of the following infections would not be consistent with the diagnosis?

A. Giardiasis
B. Pneumocystis carinii pneumonia (Correct Answer)
C. Recurrent sinusitis
D. Recurrent subcutaneous abscesses

Explanation: **Explanation:** The core concept tested here is the distinction between **Humoral (B-cell) Immunity** and **Cell-mediated (T-cell) Immunity**. **Why B is the Correct Answer:** *Pneumocystis jirovecii* (formerly *P. carinii*) is an opportunistic fungus. Defense against *Pneumocystis* depends primarily on **T-cell mediated immunity** (specifically CD4+ T-cells). Therefore, *Pneumocystis* pneumonia (PCP) is a hallmark of T-cell deficiencies (e.g., HIV/AIDS, SCID, or DiGeorge Syndrome), not isolated humoral immunodeficiency [1]. **Analysis of Incorrect Options:** * **A. Giardiasis:** Secretory IgA is essential for neutralizing parasites in the gut. Patients with humoral deficiencies, particularly **Selective IgA Deficiency** or **Common Variable Immunodeficiency (CVID)**, are highly susceptible to chronic *Giardia lamblia* infections. * **C. Recurrent Sinusitis:** B-cell defects lead to a lack of opsonizing antibodies (IgG). This results in recurrent sinopulmonary infections caused by **encapsulated bacteria** (e.g., *Streptococcus pneumoniae*, *Haemophilus influenzae*) [2]. * **D. Recurrent Subcutaneous Abscesses:** Recurrent skin and soft tissue infections by pyogenic bacteria (like *Staphylococcus aureus*) are common in patients with hypogammaglobulinemia due to impaired opsonization and phagocytosis [2]. **High-Yield Clinical Pearls for NEET-PG:** * **B-cell defects:** Present after 6 months of age (once maternal IgG wanes) with recurrent pyogenic sinopulmonary infections and Giardiasis [2]. * **T-cell defects:** Present early (infancy) with viral, fungal (*Candida*, *Pneumocystis*), and intracellular bacterial infections [1]. * **Bruton’s Agammaglobulinemia:** X-linked; characterized by absent B-cells and low levels of all immunoglobulin classes [1]. * **Selective IgA Deficiency:** The most common primary immunodeficiency; often presents with airway/GI infections or anaphylaxis during blood transfusions. **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. 165-166.

Practice by Chapter

Cells and Tissues of the Immune System

Practice Questions

Innate Immunity

Practice Questions

Adaptive Immunity

Practice Questions

Hypersensitivity Reactions

Practice Questions

Autoimmune Diseases

Practice Questions

Immunodeficiency Disorders

Practice Questions

Transplantation Immunopathology

Practice Questions

Immune Response to Infections

Practice Questions

Immunologic Laboratory Techniques

Practice Questions

Tumor Immunology

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free