Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 211: Which is the rich source of IgG?

A. Stomach
B. Visceral peritoneum
C. Small intestine (Correct Answer)
D. Large intestine

Explanation: ### Explanation **Correct Answer: C. Small intestine** The small intestine is the correct answer due to the presence of **Peyer’s patches** and the extensive **Gut-Associated Lymphoid Tissue (GALT)**. While IgA is the primary immunoglobulin secreted into the intestinal lumen (secretory IgA), the small intestine is a massive reservoir for B-cell maturation and plasma cell activity. The lamina propria of the small intestine contains a high density of plasma cells that produce **IgG** to provide systemic immunity and protect the mucosal barrier against invasive pathogens. Furthermore, the neonatal Fc receptor (FcRn) in the intestinal epithelium facilitates the transport of IgG, making the small intestine a critical site for IgG concentration and transport compared to other GI segments. **Analysis of Incorrect Options:** * **A. Stomach:** The stomach has a highly acidic environment and a relatively sparse distribution of lymphoid tissue compared to the intestines. Its primary defense is chemical rather than immunological. * **B. Visceral peritoneum:** This is a serous membrane. While it contains "milky spots" (small lymphoid aggregations), its total immunological output and IgG concentration are significantly lower than the specialized GALT of the small intestine. * **D. Large intestine:** Although the large intestine contains lymphoid follicles, the density of Peyer’s patches and the overall surface area for immune cell infiltration are significantly higher in the small intestine (specifically the ileum). **NEET-PG High-Yield Pearls:** * **IgA** is the most abundant antibody in *secretions* (tears, saliva, colostrum, gut lumen). * **IgG** is the most abundant antibody in *serum* and the only one that crosses the **placenta**. * **Peyer’s patches** are specialized lymphoid collections found primarily in the **ileum** (small intestine). * **M cells** (Microfold cells) are specialized cells in the small intestine that sample antigens from the lumen and deliver them to underlying lymphoid tissue.

Question 212: Which of the following statements about macrophages is false?

A. Macrophages can harbor mycobacteria.
B. Macrophages are derived from blood monocytes.
C. Macrophages are involved in type III hypersensitivity reactions. (Correct Answer)
D. Macrophages produce tumor necrosis factor and interleukins.

Explanation: **Explanation:** The correct answer is **C**. Type III hypersensitivity is an **immune-complex-mediated** reaction. It involves the deposition of antigen-antibody complexes in tissues, which activates the **complement system**. The primary effector cells recruited to the site are **Neutrophils**, which release lysosomal enzymes causing tissue damage (e.g., Arthus reaction, SLE, Serum sickness). While macrophages are professional phagocytes, they are the hallmark of **Type IV (Delayed-type) hypersensitivity**, not Type III. **Analysis of other options:** * **Option A:** Macrophages are the primary host cells for *Mycobacterium tuberculosis*. These bacteria survive intracellularly by inhibiting phagosome-lysosome fusion. * **Option B:** Monocytes are produced in the bone marrow, circulate in the blood for about 8 hours, and then migrate into tissues to differentiate into specific macrophages (e.g., Kupffer cells in the liver, Microglia in the CNS). * **Option D:** Macrophages are key secretory cells. Upon activation (often by LPS or IFN-γ), they produce pro-inflammatory cytokines including **TNF-α, IL-1, IL-6, and IL-12**. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** CD14 is a specific surface marker for monocytes/macrophages (acts as a receptor for LPS). * **Life Span:** Unlike neutrophils (short-lived), macrophages are long-lived and capable of division at the inflammation site. * **Granuloma:** In chronic inflammation, macrophages transform into **Epithelioid cells** under the influence of IFN-γ. * **M1 vs M2:** M1 macrophages are "pro-inflammatory" (microbicidal), while M2 are "alternative" (involved in tissue repair and anti-inflammatory responses).

Question 213: Transplantation of the host's own tissue is known as?

A. Isograft
B. Allograft
C. Xenograft
D. Autograft (Correct Answer)

Explanation: **Explanation:** The correct answer is **Autograft**. This term refers to the transplantation of tissue from one site to another within the same individual. Since the donor and recipient are the same person, the MHC (Major Histocompatibility Complex) molecules are identical, resulting in **zero risk of graft rejection**. Common clinical examples include skin grafting for burns or using the saphenous vein for coronary artery bypass grafting (CABG). **Analysis of Incorrect Options:** * **Isograft (Syngeneic graft):** This involves transplantation between genetically identical individuals, such as **monozygotic (identical) twins**. Like autografts, these are usually accepted without rejection. * **Allograft (Homograft):** This is the most common clinical transplant type, occurring between genetically different members of the **same species** (e.g., human to human). These require immunosuppression to prevent rejection due to HLA mismatch. * **Xenograft (Heterograft):** This involves transplantation between members of **different species** (e.g., pig heart valve to a human). These are prone to rapid, hyperacute rejection. **NEET-PG High-Yield Pearls:** 1. **Order of Rejection Risk:** Autograft = Isograft < Allograft < Xenograft. 2. **MHC/HLA:** The primary target of the immune response in graft rejection is the **MHC Class II** molecules on the donor's dendritic cells (Direct Pathway). 3. **Privileged Sites:** Certain areas like the **cornea, anterior chamber of the eye, and testes** have low risk of rejection because they are "immunologically privileged" (sequestered from immune surveillance).

Question 214: All of the following are categorized as secondary lymphoid organs except?

A. Lymph nodes
B. Spleen
C. Thymus (Correct Answer)
D. Subepithelial collections of lymphocytes

Explanation: ### Explanation Lymphoid organs are categorized into two types based on their function in lymphocyte development: **Primary** and **Secondary**. **1. Why Thymus is the Correct Answer:** The **Thymus** and **Bone Marrow** are **Primary Lymphoid Organs**. These are the sites where lymphocytes are produced (lymphopoiesis) and undergo antigen-independent maturation. In the thymus, T-cell progenitors from the bone marrow differentiate into mature, immunocompetent T-cells while learning self-tolerance. Since the question asks for the exception to secondary organs, the Thymus is the correct choice. **2. Analysis of Incorrect Options (Secondary Lymphoid Organs):** Secondary lymphoid organs are sites where mature lymphocytes reside, encounter antigens, and initiate an immune response (antigen-dependent proliferation). * **Lymph Nodes (Option A):** Filter lymph and are the primary site for local immune responses to tissue-borne antigens. * **Spleen (Option B):** Filters blood and is the chief site of immune responses to blood-borne antigens. * **Subepithelial collections (Option D):** These include **MALT** (Mucosa-Associated Lymphoid Tissue) such as Peyer’s patches in the intestine, tonsils, and appendix. They protect mucosal surfaces. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **B-cell Maturation:** Occurs in the Bone Marrow (Primary). * **T-cell Maturation:** Occurs in the Thymus (Primary). * **Hassall’s Corpuscles:** Characteristic histological feature of the Thymic medulla. * **DiGeorge Syndrome:** Congenital failure of the 3rd and 4th pharyngeal pouches to develop, leading to thymic aplasia and T-cell deficiency. * **Mnemonic:** Primary organs are where cells are **"Born and Trained"** (Bone marrow/Thymus); Secondary organs are where they **"Fight"** (Spleen/Nodes/MALT).

Question 215: A 24-year-old man is diagnosed with disseminated histoplasmosis after developing symptoms of fever, lymphadenopathy, hepatosplenomegaly, and pancytopenia. Which of the following is the body's major immunologic defense against histoplasmosis?

A. Immunoglobulin G (IgG) antibodies
B. Mononuclear leukocytes (Correct Answer)
C. Complement
D. Immunoglobulin M (IgM) antibody

Explanation: **Explanation:** **1. Why Mononuclear Leukocytes are Correct:** *Histoplasma capsulatum* is a **dimorphic intracellular fungus**. The primary defense against intracellular pathogens (fungi, mycobacteria, and certain protozoa) is **Cell-Mediated Immunity (CMI)**. When *Histoplasma* spores are inhaled, they are phagocytosed by alveolar macrophages. In a healthy immune system, T-helper cells (CD4+) recognize fungal antigens and release cytokines like **IFN-gamma**. This activates **mononuclear leukocytes** (macrophages and monocytes), enhancing their phagocytic and fungicidal activity to contain the infection within granulomas. In disseminated disease (as seen in this patient), a failure of this T-cell/macrophage axis allows the yeast to spread throughout the reticuloendothelial system. **2. Why Other Options are Incorrect:** * **Options A & D (IgG and IgM):** Humoral immunity (antibodies) plays a minimal role in controlling intracellular fungal infections. While antibodies are produced and can be used for diagnosis (serology), they do not provide protective clearance. * **Option C (Complement):** The complement system is vital for opsonization and lysis of extracellular bacteria, but it is ineffective against the intracellular yeast form of *Histoplasma* residing inside macrophages. **3. NEET-PG Clinical Pearls:** * **Intracellular Niche:** *Histoplasma* is the only fungus that is an **obligate intracellular** parasite of the reticuloendothelial system (found inside macrophages). * **Morphology:** On biopsy (Grocott’s Methenamine Silver stain), look for small, oval budding yeasts inside macrophages. * **Risk Factor:** Disseminated histoplasmosis is a classic **AIDS-defining illness** (usually when CD4 <150 cells/mm³). * **Mimicry:** It clinically mimics Tuberculosis (fever, weight loss, granulomas, and hepatosplenomegaly).

Question 216: Which of the following cells is primarily involved in T cell independent antigen presentation?

A. T cell
B. B cell (Correct Answer)
C. Macrophage
D. CD8+ cytotoxic T lymphocyte

Explanation: ### Explanation **Correct Option: B (B cell)** Antigens are classified into two types based on their requirement for T-cell help: **T-dependent (TD)** and **T-independent (TI)**. * **T-independent antigens** (e.g., bacterial polysaccharides, lipopolysaccharides) contain repeating epitopes that can directly cross-link multiple B-cell receptors (BCRs). * This cross-linking provides a signal strong enough to activate the **B cell** directly, leading to IgM production without the assistance of Th2 cells or MHC Class II presentation. Therefore, B cells are the primary effectors in responding to these antigens. **Why other options are incorrect:** * **A & D (T cells / CD8+ cells):** T cells cannot recognize antigens in their native form. They require antigens to be processed and presented on MHC molecules by Professional Antigen Presenting Cells (APCs). By definition, T-cell activation is "T-cell dependent." * **C (Macrophage):** While macrophages are professional APCs, their primary role is to process protein antigens and present them to T cells via MHC II. They do not undergo the direct clonal expansion and antibody secretion characteristic of the T-independent response seen in B cells. **High-Yield Clinical Pearls for NEET-PG:** * **TI-Antigen Characteristics:** Usually non-protein (polysaccharides). They do **not** produce immunological memory, do **not** undergo isotype switching (mostly IgM), and do **not** show affinity maturation. * **Anatomical Site:** The response to TI antigens occurs primarily in the **marginal zone of the spleen**. * **Clinical Relevance:** Patients with asplenia (e.g., Sickle Cell Disease) are highly susceptible to encapsulated bacteria (Streptococcus pneumoniae, H. influenzae) because they cannot mount an effective T-independent B-cell response against polysaccharide capsules. * **Vaccine Science:** Conjugate vaccines (like Hib) convert a TI antigen into a TD antigen by attaching a protein carrier, allowing for T-cell involvement and long-term memory.

Question 217: Infection with Epstein-Barr virus (EBV) results in the development of virus-specific antibodies. The pattern of these antibodies helps to stage the illness. Which statement regarding EBV-VCA (IgG) Ab is correct?

A. Appears 2 weeks to several months after onset and is present more often in atypical cases of infectious mononucleosis
B. Appears 3 to 4 weeks after onset; titers correlate with severity of clinical illness
C. Arises early in the course of the illness; detectable titers persist a lifetime (Correct Answer)
D. Appears late in the course of the disease and persists a lifetime

Explanation: **Explanation:** The diagnosis and staging of **Infectious Mononucleosis (IM)** caused by Epstein-Barr Virus (EBV) rely on the temporal appearance of specific antibodies against viral antigens. **Why Option C is Correct:** The **Viral Capsid Antigen (VCA)-IgG** antibody is the most reliable marker for EBV infection. It arises **early** (often present at the onset of clinical symptoms) and, unlike VCA-IgM, it **persists for the lifetime** of the individual. Its presence indicates that the patient has been infected with EBV at some point (either acute or past infection). **Analysis of Incorrect Options:** * **Option A & B:** VCA-IgG does not appear weeks or months later; it is usually detectable by the time the patient seeks medical attention. Furthermore, antibody titers in EBV **do not correlate with the severity** of the clinical illness. * **Option D:** This description fits **EBNA (Epstein-Barr Nuclear Antigen) antibodies**. EBNA antibodies appear late (3–4 weeks after onset, during convalescence) and persist for life. Their presence helps rule out acute infection. **High-Yield Clinical Pearls for NEET-PG:** 1. **Acute Infection Profile:** Positive VCA-IgM, Positive VCA-IgG, and **Negative EBNA**. 2. **Past Infection Profile:** Positive VCA-IgG and **Positive EBNA** (VCA-IgM is negative). 3. **Heterophile Antibodies (Monospot Test):** These are non-specific IgM antibodies that agglutinate sheep/horse RBCs. They are the first-line screening test but can be negative in children <5 years. 4. **Atypical Lymphocytes:** On a peripheral smear, look for "Downey cells" (activated T-cells), which are characteristic of IM.

Question 218: Thymic hypoplasia is seen in which of the following conditions?

A. DiGeorge syndrome
B. Common variable immunodeficiency
C. Ataxia telangiectasia
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above**. Thymic hypoplasia refers to the underdevelopment of the thymus, leading to impaired T-cell maturation and cell-mediated immunity. 1. **DiGeorge Syndrome (Option A):** This is the classic cause of thymic hypoplasia. It results from a microdeletion on chromosome **22q11**, leading to the failure of the **3rd and 4th pharyngeal pouches** to develop. This manifests as the CATCH-22 mnemonic: Cardiac defects, Abnormal facies, **Thymic hypoplasia**, Cleft palate, and Hypocalcemia (due to parathyroid hypoplasia). 2. **Ataxia Telangiectasia (Option C):** This is an autosomal recessive DNA repair defect (ATM gene). While primarily known for cerebellar ataxia and oculocutaneous telangiectasia, it also involves **thymic hypoplasia/dysplasia**, leading to combined immunodeficiency and an increased risk of malignancies. 3. **Common Variable Immunodeficiency (CVID) (Option B):** While CVID is primarily characterized by hypogammaglobulinemia (B-cell defect), a significant subset of patients (up to 20-30%) exhibit T-cell abnormalities, including reduced thymic output and functional thymic impairment, which can manifest as varying degrees of thymic atrophy or hypoplasia. **NEET-PG High-Yield Pearls:** * **Chest X-ray finding:** The absence of a "Thymic Shadow" in a neonate is a classic sign of DiGeorge Syndrome or SCID. * **Nezelof Syndrome:** An autosomal recessive condition characterized by thymic hypoplasia with normal or near-normal immunoglobulin levels. * **SCID (Severe Combined Immunodeficiency):** Also presents with a vestigial/hypoplastic thymus, but is not listed as a standalone option here. * **Hassall’s Corpuscles:** These are characteristically absent or reduced in cases of thymic dysplasia/hypoplasia.

Question 219: What is true about IgM?

A. Fixes complement (Correct Answer)
B. Increased in primary response
C. The Fab region is composed of the variable region
D. The Fc region is antibody-binding

Explanation: ### Explanation **Correct Option: A (Fixes complement)** IgM is the most potent activator of the **Classical Complement Pathway**. It exists primarily as a pentamer (connected by a J-chain), providing multiple binding sites for the C1q component of the complement cascade. A single pentameric IgM molecule bound to an antigen can initiate the cascade, whereas IgG requires at least two molecules in close proximity. **Analysis of Incorrect Options:** * **B. Increased in primary response:** While IgM is the *first* antibody produced in a primary immune response, the statement is technically incomplete or less "true" than its complement-fixing ability in a comparative sense. However, in many exam contexts, IgM is the hallmark of the **acute/primary phase**, while IgG dominates the secondary (anamnestic) response. * **C. The Fab region is composed of the variable region:** This is partially incorrect. The Fab (Fragment antigen-binding) region is composed of **both** the entire variable region (VH and VL) and the first constant region (CH1 and CL). * **D. The Fc region is antibody-binding:** This is incorrect. The **Fab** region binds to the antigen (antibody-binding), while the **Fc** (Fragment crystallizable) region binds to host cell receptors (Fc receptors) and mediates effector functions like complement activation and opsonization. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** IgM is a **pentamer** in serum (10 binding sites) but a **monomer** on the surface of B-cells (BCR). * **Molecular Weight:** It is the largest immunoglobulin (**"Millionaire Molecule"**), preventing it from crossing the placenta. * **Clinical Significance:** Presence of **IgM in a newborn** indicates intrauterine infection (e.g., TORCH), as maternal IgM cannot cross the placenta. * **Agglutination:** Due to its high valency, IgM is the most efficient antibody for agglutination (e.g., ABO blood grouping).

Question 220: Papain acts on gamma globulin to form which of the following?

A. 2 Fc fragments
B. 2 Fab fragments (Correct Answer)
C. 1 Fab fragment
D. None of the above

Explanation: **Explanation:** The structure of an immunoglobulin (IgG) molecule consists of two heavy chains and two light chains held together by disulfide bonds. The enzymatic cleavage of this molecule is a high-yield topic in immunology. **Why Option B is Correct:** When the enzyme **Papain** acts on a gamma globulin (IgG) molecule, it cleaves the heavy chains at the **hinge region**, specifically *above* the interchain disulfide bonds. This proteolytic cleavage results in three distinct fragments: 1. **Two Fab fragments** (Fragment Antigen Binding): Each fragment consists of one light chain and a portion of one heavy chain. These fragments are monovalent, meaning they can bind to an antigen but cannot precipitate or cross-link them. 2. **One Fc fragment** (Fragment Crystallizable): This consists of the remaining portions of the two heavy chains held together. It is responsible for biological effector functions like complement fixation and opsonization. **Why Other Options are Incorrect:** * **Option A:** Papain produces only **one** Fc fragment, not two. The two heavy chain segments in the Fc region remain joined by disulfide bonds. * **Option C:** Papain produces **two** separate Fab fragments because it cuts above the site where the heavy chains are linked to each other. **NEET-PG High-Yield Pearls:** * **Pepsin Digestion:** Unlike Papain, Pepsin cleaves *below* the hinge region. This results in **one F(ab')₂ fragment** (bivalent, can precipitate antigens) and degraded Fc subfragments. * **Mercaptoethanol:** This reducing agent breaks all disulfide bonds, resulting in four separate polypeptide chains (2 Heavy, 2 Light). * **Memory Aid:** **Pa**pain = **Pa**rted (splits the two Fabs apart); **Pe**psin = **Pe**rmanent (keeps the two Fabs together as one unit).

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Immunology — MCQs

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