Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following immunoglobulins is increased in primary bacterial infections?

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

Explanation: **Explanation:** **Why IgM is the correct answer:** IgM is the first immunoglobulin class produced in response to a primary infection (the **primary immune response**). It is a pentamer with 10 antigen-binding sites, making it highly efficient at agglutination and complement activation via the classical pathway. Because it is the earliest antibody to appear after exposure to a new bacterial antigen, its presence in high titers typically indicates an acute or recent infection. **Analysis of Incorrect Options:** * **IgG:** This is the most abundant immunoglobulin in the serum but is characteristic of the **secondary (anamnestic) immune response**. It increases significantly upon re-exposure to an antigen or in the later stages of a primary infection. It is the only antibody that crosses the placenta. * **IgA:** This is the primary "secretory" antibody. It is found predominantly in body secretions (saliva, tears, colostrum, and GI/respiratory mucus) and provides local mucosal immunity rather than being the primary systemic marker for acute bacterial infection. * **IgD:** This antibody acts primarily as a B-cell surface receptor. Its exact systemic function is less defined, and it does not increase significantly during primary bacterial infections. **High-Yield NEET-PG Pearls:** * **IgM:** Largest immunoglobulin (Pentamer); cannot cross the placenta; first to appear phylogenetically and ontogenetically (produced by the fetus). * **IgG:** Longest half-life (approx. 23 days); mediates Type II and III hypersensitivity. * **IgE:** Lowest serum concentration; mediates Type I hypersensitivity and provides defense against helminthic infections. * **Isotype Switching:** The process where B-cells switch from producing IgM to IgG/IgA/IgE, stimulated by cytokines from T-helper cells.

Question 32: Which of the following is a live attenuated vaccine?

A. Salk polio vaccine
B. Sabin polio vaccine (Correct Answer)
C. Rabies vaccine
D. KFD vaccine

Explanation: **Explanation:** The correct answer is **Sabin polio vaccine (Option B)**. Vaccines are broadly classified into live attenuated, killed (inactivated), toxoids, and subunit vaccines. 1. **Why Sabin is correct:** The Sabin vaccine is an **Oral Polio Vaccine (OPV)** containing live-attenuated viruses. It mimics natural infection by inducing both systemic immunity (IgG) and local mucosal immunity (secretory IgA) in the gut. This prevents the wild virus from multiplying in the intestines, thereby stopping community transmission. 2. **Why other options are incorrect:** * **Salk polio vaccine (Option A):** This is an **Inactivated Polio Vaccine (IPV)** administered via injection. It induces systemic IgG but provides minimal mucosal IgA. * **Rabies vaccine (Option B):** Modern rabies vaccines (like PCECV or HDCV) are **killed/inactivated** vaccines. * **KFD vaccine (Option D):** The vaccine for Kyasanur Forest Disease (KFD) is a **formalin-inactivated** (killed) vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Live Vaccines:** "**Rome Is My Best Place**" (**R**ubella, **O**PV, **M**easles/Mumps, **E**nteric fever (Ty21a), **I**nfluenza (Intranasal), **M**arburg/Yellow fever, **B**CG, **P**oliosabin). * **VAPP & VDPV:** A rare complication of the Sabin vaccine is Vaccine-Associated Paralytic Poliomyelitis (VAPP) or Vaccine-Derived Poliovirus (VDPV), which does not occur with the Salk (killed) vaccine. * **Contraindication:** Live vaccines are generally contraindicated in pregnancy and immunocompromised states (except HIV patients before the symptomatic stage).

Question 33: Class I histocompatibility antigens are expressed on all of the following cell types except?

A. Macrophage
B. Lymphocyte (Correct Answer)
C. Platelets
D. Red Blood Cells

Explanation: **Explanation:** The expression of Major Histocompatibility Complex (MHC) molecules is a fundamental concept in immunology. **MHC Class I molecules** (HLA-A, B, and C) are found on the surface of **all nucleated cells** in the body, as well as on **platelets**. Their primary role is to present endogenous antigens to CD8+ T-cytotoxic cells. **Why Option D (Red Blood Cells) is the correct answer:** Mature Red Blood Cells (RBCs) are **non-nucleated**. Since they lack a nucleus and the necessary protein synthesis machinery (ribosomes/endoplasmic reticulum) to process and display these surface glycoproteins, they do not express MHC Class I antigens. **Analysis of Incorrect Options:** * **A. Macrophages:** These are nucleated cells and professional Antigen Presenting Cells (APCs). They express **both** MHC Class I and MHC Class II. * **B. Lymphocytes:** Both B and T lymphocytes are nucleated and express MHC Class I. (Note: B-cells also express MHC Class II). * **C. Platelets:** Although platelets are anuclear fragments of megakaryocytes, they are a notable **exception** to the "nucleated cell" rule. They do express MHC Class I antigens on their surface, which is clinically significant in platelet transfusion refractoriness. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I:** Found on all nucleated cells + Platelets. (Presents to CD8+ T cells). * **MHC Class II:** Found only on Professional APCs (Macrophages, B-cells, Dendritic cells) and thymic epithelial cells. (Presents to CD4+ T cells). * **MHC Class III:** Includes components of the complement system (C2, C4) and TNF. * **Human exception:** The only nucleated cell that lacks MHC Class I is the **villous trophoblast**, which helps prevent maternal immune rejection of the fetus.

Question 34: Soluble components of the complement system account for approximately what concentration of total serum protein?

A. 1-2 mg/ml
B. 3-4 mg/ml (Correct Answer)
C. 5-7 mg/ml
D. 7-8 mg/ml

Explanation: **Explanation:** The complement system is a vital component of innate immunity, consisting of more than 30 soluble and cell-bound proteins. In healthy individuals, these soluble proteins circulate in the blood as inactive precursors (zymogens). Collectively, they constitute approximately **3 to 4 mg/ml** of the total serum protein, which represents about **5% to 10% of the total serum globulin fraction**. * **Why Option B is correct:** The cumulative concentration of all complement components (C1 through C9, along with regulatory factors like Factor B and D) typically falls within the 3–4 mg/ml range. Among these, **C3 is the most abundant** complement protein in the serum (approx. 1.2 mg/ml), followed by C4 and C1q. * **Why Options A, C, and D are incorrect:** * **1-2 mg/ml (A):** This is too low; the concentration of C3 alone accounts for nearly this entire range. * **5-7 mg/ml (C) and 7-8 mg/ml (D):** These values overestimate the physiological concentration. While complement levels are "acute-phase reactants" and can rise during inflammation, the baseline physiological concentration remains lower. **High-Yield Clinical Pearls for NEET-PG:** * **Most abundant component:** C3 (often measured clinically to screen for consumption/deficiency). * **Least abundant component:** C2 (often the rate-limiting step in the classical pathway). * **Heat Lability:** Complement proteins are heat-labile; they are inactivated by heating serum at **56°C for 30 minutes**. * **Site of Synthesis:** Most complement proteins are synthesized in the **liver**, though macrophages and intestinal epithelium also contribute.

Question 35: Which of the following factors is responsible for deciding whether an antibody or immunoglobulin will remain membrane bound or get secreted?

A. RNA Splicing
B. Class Switching
C. Differential RNA Processing (Correct Answer)
D. Allelic Exclusion

Explanation: **Explanation:** The decision of whether an immunoglobulin (Ig) is produced as a **membrane-bound receptor** (on B-cells) or a **secreted antibody** (by plasma cells) is determined by **Differential RNA Processing** (specifically, alternative polyadenylation and splicing). 1. **Why it is correct:** The heavy chain gene contains two potential polyadenylation sites. * If the first site is used, the mRNA excludes the hydrophobic transmembrane segment, resulting in a shorter protein that is **secreted**. * If the second site is used, the mRNA includes the hydrophobic C-terminal sequence that anchors the protein to the cell membrane. This process occurs *after* transcription but *before* translation, allowing a single B-cell to switch from a surface receptor to a secreted product without changing its DNA. 2. **Why other options are incorrect:** * **RNA Splicing (A):** While a component of processing, "Differential RNA Processing" is the more precise term as it encompasses the choice of polyadenylation sites which dictates the final splice form. * **Class Switching (B):** This involves DNA recombination to change the constant region (e.g., IgM to IgG). It changes the *isotype* and function, but not whether the Ig is membrane-bound or secreted. * **Allelic Exclusion (D):** This process ensures that a B-cell expresses an antigen receptor from only one of the two inherited alleles (parental or maternal), ensuring monoclonal specificity. **High-Yield Clinical Pearls for NEET-PG:** * **Membrane-bound Ig:** Found on naive B-cells (predominantly IgM and IgD). * **Secreted Ig:** Produced by terminal differentiation of B-cells into **Plasma cells**. * **Key Concept:** Somatic hypermutation and Class switching happen at the **DNA level**, whereas the choice between membrane-bound vs. secreted Ig happens at the **RNA level**.

Question 36: Which serum immunoglobulin constitutes 80% of immunoglobulins in our body?

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

Explanation: **Explanation:** **IgG** is the most abundant class of immunoglobulin in the human body, constituting approximately **75–80%** of the total serum pool. It is the primary antibody of the secondary immune response and is uniquely characterized by its ability to cross the placenta, providing passive immunity to the fetus. **Why the other options are incorrect:** * **IgM (5–10%):** It is the largest immunoglobulin (pentamer) and the first to appear in response to an initial antigen exposure (primary response). It is primarily intravascular due to its high molecular weight. * **IgA (10–15%):** It is the second most common serum antibody but is the **most abundant in secretions** (colostrum, saliva, tears, and respiratory/GI mucus), where it provides mucosal immunity. * **IgD (<1%):** It is found in trace amounts in serum and primarily acts as a B-cell surface receptor for antigen recognition. **High-Yield NEET-PG Pearls:** * **Abundance Mnemonic:** Remember the order of serum concentration as **GAMDE** (IgG > IgA > IgM > IgD > IgE). * **Placental Transfer:** Only IgG crosses the placenta (specifically subclasses IgG1, IgG3, and IgG4). * **Half-life:** IgG has the longest half-life (approx. 23 days), making it useful for long-term immunity. * **Complement Activation:** IgM is the most efficient at activating the classical complement pathway, followed by IgG.

Question 37: What is the most abundant complement protein in the body?

A. C1
B. C2
C. C3 (Correct Answer)
D. C4

Explanation: **Explanation:** **C3** is the most abundant complement protein in human serum, with a normal concentration of approximately **1.2 mg/mL**. It serves as the central hub of the complement system because all three activation pathways (Classical, Alternative, and Lectin) converge at the step of C3 cleavage. Its high concentration is physiologically necessary because C3b (its cleavage product) acts as a major opsonin and is required to form the C5 convertase, which initiates the Membrane Attack Complex (MAC). **Analysis of Incorrect Options:** * **C1:** This is the first component of the classical pathway. While large in molecular size (a complex of C1q, C1r, and C1s), its serum concentration is significantly lower than C3. * **C2:** This is the least abundant complement component in the serum. It is often the rate-limiting factor in the classical pathway activation. * **C4:** Although C4 is present in higher concentrations than C2, it ranks below C3. It is primarily involved in the classical and lectin pathways. **Clinical Pearls for NEET-PG:** * **Central Molecule:** C3 is the most important component; its deficiency leads to severe, recurrent pyogenic infections (e.g., *S. pneumoniae*, *H. influenzae*). * **C3b Function:** "C3**b** **B**inds" to bacteria (Opsonization). * **C3a/C5a Function:** These are **Anaphylatoxins** (trigger mast cell degranulation). C5a is also a potent chemoattractant for neutrophils. * **Diagnostic Marker:** Low levels of C3 and C4 are used clinically to monitor disease activity in Systemic Lupus Erythematosus (SLE) and Post-Streptococcal Glomerulonephritis (PSGN).

Question 38: Active immunity is not acquired by which of the following?

A. Infection
B. Vaccination
C. Immunoglobulin transfer (Correct Answer)
D. Subclinical infection

Explanation: **Explanation:** The core concept tested here is the distinction between **Active** and **Passive** immunity. **Active Immunity** occurs when an individual’s own immune system is stimulated to produce antibodies and memory cells following exposure to an antigen. This process takes time to develop but provides long-lasting protection. * **Infection (Option A) and Subclinical Infection (Option D):** These represent **Natural Active Immunity**. Whether the person shows symptoms (clinical) or remains asymptomatic (subclinical), the body encounters the live pathogen and mounts an immune response. * **Vaccination (Option B):** This represents **Artificial Active Immunity**. Antigens (killed, attenuated, or toxoids) are introduced to trigger the immune system without causing the full disease. **Why Immunoglobulin Transfer is the Correct Answer:** **Immunoglobulin transfer (Option C)** is a form of **Passive Immunity**. In this case, pre-formed antibodies are directly transferred to the individual. The recipient’s immune system remains "passive" and does not produce its own antibodies or memory cells. This provides immediate but temporary protection (e.g., IVIG, Tetanus Antitoxin, or natural transfer via the placenta/colostrum). **High-Yield NEET-PG Pearls:** * **Memory:** Active immunity produces immunological memory; Passive immunity does **not**. * **Negative Phase:** Active immunity may have a "negative phase" (temporary dip in resistance) immediately after antigen injection; Passive immunity has no negative phase. * **Natural Passive:** IgG crossing the placenta (only antibody that can) and IgA in colostrum. * **Artificial Passive:** Administration of antisera (e.g., Anti-rabies serum, Anti-tetanus serum).

Question 39: Common variable immunodeficiency is due to which of the following?

A. Absent B cells
B. Reduced number of B cells
C. Defective B cell differentiation (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Common Variable Immunodeficiency (CVID)** is a primary immunodeficiency characterized by low levels of serum immunoglobulins (IgG, IgA, and often IgM) and a failure to produce specific antibodies following vaccination or infection. **Why Option C is correct:** The hallmark of CVID is not the absence of B cells, but rather a **defect in B cell differentiation** into antibody-secreting plasma cells and memory B cells. While patients usually have a normal or slightly reduced number of peripheral B cells, these cells are functionally immature and cannot undergo terminal differentiation. This leads to hypogammaglobulinemia. **Why other options are incorrect:** * **Option A & B:** Absent or severely reduced B cells (typically <2%) are characteristic of **X-linked Agammaglobulinemia (Bruton’s)**, caused by a mutation in the BTK gene. In CVID, B cell numbers are usually within the normal range or only mildly decreased. * **Option D:** Since the primary pathology is a functional maturation defect rather than a quantitative loss of B cells, "All of the above" is incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Age of Onset:** Unlike Bruton’s (infancy), CVID typically presents in the **2nd–3rd decade** of life (bimodal peaks: 1-5 years and 18-30 years). * **Clinical Features:** Recurrent sinopulmonary infections (H. influenzae, S. pneumoniae), increased risk of **Giardiasis**, and autoimmune disorders. * **Malignancy Risk:** Significant predisposition to **Non-Hodgkin Lymphoma** and Gastric Carcinoma. * **Diagnosis:** Low IgG (at least 2 SD below mean) plus low IgA and/or IgM, with poor response to vaccines.

Question 40: A 45-year-old homeless man presents with chronic cough and a cavitary lesion of the lungs. His sputum is positive for acid-fast bacilli. Which of the following represents the primary peripheral form of defense by which his body fights this infection?

A. Antibody-mediated immunity
B. Cell-mediated hypersensitivity (Correct Answer)
C. IgA-mediated hypersensitivity
D. IgE-mediated hypersensitivity

Explanation: **Explanation:** The clinical presentation of chronic cough, cavitary lung lesions, and acid-fast bacilli (AFB) in sputum is diagnostic of **Pulmonary Tuberculosis** caused by *Mycobacterium tuberculosis*. **1. Why the correct answer is right:** *Mycobacterium tuberculosis* is an **obligate intracellular pathogen** that survives within macrophages. Because the bacteria are sequestered inside cells, humoral immunity (antibodies) cannot reach them. The body relies on **Type IV Hypersensitivity (Delayed-type/Cell-mediated hypersensitivity)**. In this process, CD4+ T-cells (Th1) recognize mycobacterial antigens and release **Interferon-gamma (IFN-γ)**, which activates macrophages to kill the intracellular bacteria. The formation of a granuloma (the "cavitary lesion") is the hallmark of this cell-mediated immune response. **2. Why incorrect options are wrong:** * **Option A:** Antibody-mediated immunity (Humoral) is effective against extracellular bacteria and toxins. It plays a negligible role in controlling TB. * **Option C:** IgA is primarily involved in mucosal immunity (preventing attachment at surfaces), but it does not clear an established intracellular infection. * **Option D:** IgE-mediated hypersensitivity (Type I) is involved in allergic reactions (asthma, anaphylaxis) and defense against helminthic parasites. **3. NEET-PG High-Yield Pearls:** * **Key Cytokine:** IFN-γ is the most critical cytokine for activating macrophages in TB. * **Diagnostic Test:** The Mantoux (PPD) test is a classic clinical example of a Type IV hypersensitivity reaction. * **Histology:** Look for **Langhans giant cells** (fused activated macrophages) and **caseating necrosis** in TB granulomas. * **Gold Standard:** Culture on **Lowenstein-Jensen (LJ) medium** remains the traditional gold standard, though NAAT (CBNAAT/GeneXpert) is the preferred initial test.

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Complement System

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Cytokines and Chemokines

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Hypersensitivity Reactions

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Autoimmunity and Autoimmune Diseases

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Immunodeficiency Disorders

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Immunology — MCQs

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