Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 661: The process of coating a particle, such as a microbe, to target it for phagocytosis is called?

A. Opsonization (Correct Answer)
B. Priming
C. Activation
D. None of the above

Explanation: ### Explanation **Correct Option: A. Opsonization** Opsonization is the process by which foreign particles (like bacteria) are coated by specific proteins called **opsonins** to make them more "palatable" and easily recognizable by phagocytic cells (neutrophils and macrophages). Phagocytes possess surface receptors for these opsonins, allowing them to bind and engulf the pathogen efficiently. The two most important opsonins in the human body are: 1. **IgG antibody:** Specifically the Fc portion. 2. **C3b:** A fragment of the complement system (the most potent opsonin). **Why Incorrect Options are Wrong:** * **B. Priming:** This refers to the initial exposure of the immune system to an antigen, which prepares it for a swifter and stronger secondary response. In cellular biology, it can also refer to the "first signal" that makes a cell (like a macrophage) more sensitive to a second stimulus. * **C. Activation:** This is a broad term describing the transition of an immune cell from a resting state to a functional, effector state (e.g., T-cell activation or Macrophage activation by Interferon-gamma). It is the *result* of signaling, not the specific process of coating a microbe. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Ig**G** and C3**b** make bacteria **G**ood to **B**ite." * **Spleen’s Role:** The spleen is the primary site for opsonization of **encapsulated organisms** (e.g., *S. pneumoniae, H. influenzae, N. meningitidis*). Patients with asplenia are at high risk for overwhelming post-splenectomy infection (OPSI) due to defective opsonization. * **Lab Correlation:** The **Quellung Reaction** (capsular swelling) is a laboratory application of opsonization used to identify encapsulated bacteria.

Question 662: Chediak-Higashi syndrome is associated with a defect in:

A. Lysosome fusion (Correct Answer)
B. T-cells
C. B-cells
D. Complement

Explanation: **Explanation:** **Chediak-Higashi Syndrome (CHS)** is a rare autosomal recessive immunodeficiency caused by a mutation in the **LYST (Lysosomal Trafficking Regulator) gene**. This mutation leads to a defect in **microtubule-dependent vesicle trafficking**, which prevents the fusion of phagosomes with lysosomes. This results in the formation of pathognomonic **giant azurophilic granules** in neutrophils, as organelles cannot be properly distributed or fused. * **Why Option A is Correct:** The core defect is the failure of **phagosome-lysosome fusion**. Even though bacteria are ingested by neutrophils, they cannot be killed because the digestive enzymes in the lysosomes never reach the phagosome. * **Why Options B & C are Incorrect:** CHS is primarily a defect of the innate immune system (phagocytes) and intracellular trafficking, not a primary deficiency of T-cell (Cell-mediated) or B-cell (Humoral) lineage development. * **Why Option D is Incorrect:** Complement deficiencies (e.g., C3 or C5-C9) lead to defects in opsonization or MAC formation, but do not involve intracellular granule trafficking. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Partial oculocutaneous **albinism**, recurrent pyogenic infections (Staph/Strep), and peripheral neuropathy. * **Diagnosis:** Peripheral blood smear showing **giant granules** in neutrophils and platelets. * **Associated Feature:** Mild coagulation defects due to lack of dense granules in platelets. * **The "Accelerated Phase":** A life-threatening hemophagocytic lymphohistiocytosis (HLH)-like syndrome often triggered by EBV.

Question 663: ABO antibodies are primarily which immunoglobulin class?

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

Explanation: **Explanation:** The correct answer is **IgM**. ABO blood group antibodies (Anti-A and Anti-B) are considered **"natural antibodies"** because they are produced without prior exposure to foreign red blood cells, likely as a cross-reaction to environmental antigens (gut flora). These antibodies are primarily of the **IgM class**. Due to their pentameric structure, they are highly efficient at fixing complement and causing intravascular hemolysis, which is why ABO incompatibility leads to severe immediate transfusion reactions. **Analysis of Options:** * **IgG (Option A):** While ABO antibodies are primarily IgM, **Rh antibodies** (Anti-D) are primarily IgG. Additionally, in individuals with **Type O blood**, the anti-A and anti-B antibodies can sometimes be of the IgG class, which is clinically significant as IgG can cross the placenta. * **IgA (Option B):** IgA is the primary immunoglobulin found in secretions (mucosal immunity). While trace amounts may be found in serum, it is not the primary class for ABO antibodies. * **IgD (Option D):** IgD functions mainly as an antigen receptor on the surface of B-cells; it does not circulate in significant quantities as a functional antibody in the ABO system. **High-Yield Clinical Pearls for NEET-PG:** * **Cold vs. Warm:** IgM antibodies (like ABO) are "Cold Agglutinins" (react best at 4°C), whereas IgG antibodies (like Rh) are "Warm Agglutinins" (react best at 37°C). * **Placental Transfer:** Only IgG crosses the placenta. This is why ABO incompatibility rarely causes severe Hemolytic Disease of the Newborn (HDN) in Type A or B mothers (who have IgM), but can occur in Type O mothers (who may have IgG anti-A/B). * **Size Matters:** IgM is a large pentamer and cannot cross the placental barrier, whereas IgG is a small monomer.

Question 664: Which of the following is an example of a type 3 hypersensitivity reaction?

A. Goodpasture syndrome
B. Serum sickness (Correct Answer)
C. Autoimmune hemolytic anemia (AHA)
D. Asthma

Explanation: **Explanation:** Hypersensitivity reactions are classified based on the underlying immune mechanism. **Type 3 Hypersensitivity** is mediated by **Immune Complexes** (Antigen-Antibody complexes). These complexes circulate in the blood and deposit in tissues (like joints, kidneys, and blood vessels), triggering complement activation and neutrophil recruitment, leading to tissue damage. **Why Serum Sickness is Correct:** Serum sickness is the classic systemic example of Type 3 hypersensitivity. It occurs when foreign proteins (antigens) are injected, leading to the formation of soluble immune complexes that deposit in various organs. Clinical features typically include fever, rash, polyarthritis, and glomerulonephritis. **Analysis of Incorrect Options:** * **Goodpasture Syndrome (Option A):** This is a **Type 2** (Cytotoxic) reaction. It involves antibodies (Anti-GBM) binding directly to fixed antigens on the basement membranes of lungs and kidneys. * **Autoimmune Hemolytic Anemia (Option C):** This is also a **Type 2** reaction. Antibodies bind to antigens on the surface of red blood cells, leading to their destruction via the complement system or phagocytosis. * **Asthma (Option D):** This is a **Type 1** (Immediate) reaction. It is mediated by IgE antibodies causing mast cell degranulation upon exposure to allergens. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity:** **ACID** (Type 1: **A**naphylactic/Atopic; Type 2: **C**ytotoxic; Type 3: **I**mmune Complex; Type 4: **D**elayed). * **Type 3 Examples:** SLE, Post-Streptococcal Glomerulonephritis (PSGN), Arthus reaction (local), and Rheumatoid Arthritis. * **Key Mediator:** Complement components (C3a, C4a, C5a) are consumed, leading to low serum complement levels during active Type 3 disease.

Question 665: All of the following statements are true regarding Bare lymphocyte syndrome, except?

A. Autosomal recessive inheritance
B. Defect in b2-Microglobulin
C. MHC overexpression (Correct Answer)
D. MHC non-expression

Explanation: **Bare Lymphocyte Syndrome (BLS)** is a rare form of Severe Combined Immunodeficiency (SCID) characterized by the failure of lymphocytes to express Major Histocompatibility Complex (MHC) molecules on their surface. ### Explanation of Options: * **MHC Overexpression (Correct Answer):** This statement is false. BLS is defined by the **absence or severe reduction** of MHC molecules. Without these molecules, T-cells cannot "see" antigens, leading to a profound failure of the adaptive immune system. * **Autosomal Recessive Inheritance:** Most forms of BLS (both Type I and Type II) follow an autosomal recessive pattern. * **Defect in $\beta_2$-Microglobulin:** This is a classic cause of **BLS Type I (MHC Class I deficiency)**. Mutations in the TAP1, TAP2, or TAPBP genes prevent the transport of peptides into the endoplasmic reticulum, or mutations in $\beta_2$-microglobulin prevent the stable assembly of the MHC Class I molecule. * **MHC Non-expression:** This is the hallmark of the disease. In **BLS Type II (MHC Class II deficiency)**, there is a defect in the transcription factors (like CIITA or RFX) required to turn on MHC II genes, leading to a lack of MHC II expression on Antigen Presenting Cells (APCs). ### High-Yield Clinical Pearls for NEET-PG: * **Type I BLS:** Deficiency of **MHC Class I**. Presents with recurrent bacterial respiratory infections and chronic lung disease. * **Type II BLS:** Deficiency of **MHC Class II**. More common and more severe. It leads to a lack of CD4+ T-cell activation, mimicking SCID. * **Diagnostic Clue:** Normal numbers of B and T cells, but they are non-functional due to the lack of "antigen presentation" machinery. * **Treatment:** Hematopoietic stem cell transplantation (HSCT) is the definitive treatment.

Question 666: Which statement about macrophages is FALSE?

A. They help in processing and presenting antigen to immunocompetent cells.
B. They act as mediators in certain cell-mediated immune reactions.
C. They can have Fc receptors.
D. They can produce immunoglobulins. (Correct Answer)

Explanation: ### Explanation **Why Option D is the Correct Answer (The False Statement):** The production of **immunoglobulins (antibodies)** is the exclusive function of **B-lymphocytes** and their differentiated forms, **plasma cells**. Macrophages are part of the innate immune system and do not possess the genetic machinery (recombination-activating genes) required to synthesize antibodies. **Analysis of Other Options:** * **Option A (Antigen Processing/Presentation):** Macrophages are professional **Antigen-Presenting Cells (APCs)**. They ingest exogenous antigens, process them into peptides, and present them via **MHC Class II** molecules to Helper T-cells (CD4+). * **Option B (Mediators of CMI):** Macrophages are crucial effectors in Cell-Mediated Immunity (CMI). Upon activation by **Interferon-gamma (IFN-γ)** from Th1 cells, they become highly phagocytic and release cytokines (IL-1, IL-6, TNF-α) that mediate inflammation and delayed-type hypersensitivity. * **Option C (Fc Receptors):** Macrophages express **Fc receptors** (specifically for IgG) on their surface. This allows them to recognize and phagocytose opsonized pathogens (a process called opsonization). **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** Macrophages originate from blood **monocytes**. * **Tissue-Specific Names:** Kupffer cells (Liver), Microglia (CNS), Alveolar macrophages/Dust cells (Lungs), Mesangial cells (Kidney), and Osteoclasts (Bone). * **Secretions:** They produce **Interleukin-1 (IL-1)**, which acts as an endogenous pyrogen (induces fever). * **Markers:** CD14 and CD68 are common surface markers used to identify macrophages in pathology.

Question 667: Which immunoglobulin has the longest half-life?

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

Explanation: **Explanation:** The correct answer is **IgG**. **Why IgG is correct:** IgG is the most abundant immunoglobulin in the serum (constituting about 75-80% of the total pool) and possesses the longest half-life, averaging **23 days**. This longevity is attributed to its interaction with the **neonatal Fc receptor (FcRn)**. Unlike other antibodies, IgG is salvaged from degradation within endothelial cells and monocytes by FcRn, which recycles it back into the circulation. This mechanism is also responsible for the passive transfer of IgG across the placenta, providing immunity to the newborn. **Why the other options are incorrect:** * **IgA:** Has a half-life of approximately **6–8 days**. It is the primary antibody in secretions (mucosal immunity) but does not benefit from the FcRn recycling mechanism. * **IgM:** Has a half-life of approximately **5 days**. It is the first antibody produced in a primary immune response and exists mainly as a large pentamer, which is cleared more rapidly from the circulation. * **IgE:** Has the shortest half-life, approximately **2–3 days** in the serum, as it binds rapidly and with high affinity to mast cells and basophils. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Serum Concentration:** **GAMDE** (IgG > IgA > IgM > IgD > IgE). * **Placental Transfer:** IgG is the **only** immunoglobulin that crosses the placenta (IgG1, G3, and G4 specifically). * **Complement Activation:** IgM is the most efficient activator of the classical complement pathway, followed by IgG. * **Heat Lability:** IgE is unique for being heat-labile (inactivated at 56°C for 30 minutes).

Question 668: What is the most common primary immunodeficiency?

A. Common variable immunodeficiency
B. Isolated IgA immunodeficiency (Correct Answer)
C. Wiskott-Aldrich syndrome
D. AIDS

Explanation: **Explanation:** **Isolated (Selective) IgA Deficiency** is the most common primary immunodeficiency disorder worldwide, with an estimated prevalence of approximately 1 in 600 individuals in Western populations. It is characterized by serum IgA levels less than 7 mg/dL with normal levels of IgG and IgM. The underlying pathology involves a failure of B-cells to differentiate into IgA-secreting plasma cells. **Analysis of Options:** * **Option A: Common Variable Immunodeficiency (CVID):** While it is the most common *clinically significant* (symptomatic) antibody deficiency, its overall prevalence is much lower than IgA deficiency. * **Option C: Wiskott-Aldrich Syndrome:** This is a rare X-linked recessive disorder characterized by the triad of eczema, thrombocytopenia, and recurrent infections. * **Option D: AIDS:** This is a **secondary** (acquired) immunodeficiency caused by HIV. The question specifically asks for a **primary** (genetic/congenital) immunodeficiency. **High-Yield Clinical Pearls for NEET-PG:** 1. **Asymptomatic Nature:** Most patients are asymptomatic, but those who are symptomatic present with recurrent sinopulmonary infections and diarrhea (Giardiasis). 2. **Blood Transfusion Risk:** A critical complication is **Anaphylaxis during blood transfusion**. This occurs because patients develop anti-IgA antibodies; therefore, they must receive washed RBCs or IgA-deficient blood products. 3. **Associations:** Strongly associated with autoimmune diseases (SLE, Rheumatoid Arthritis) and Celiac disease (false-negative IgA anti-transglutaminase tests are common). 4. **False Positive:** Can cause a false positive pregnancy test in some assays due to heterophile antibodies.

Question 669: Diagnosis of X-linked agammaglobulinemia should be suspected if which of the following is present?

A. Absent tonsils and no palpable lymph nodes on physical examination (Correct Answer)
B. Female sex
C. High isohemagglutinin titers
D. Low CD3 count

Explanation: **Explanation:** **X-linked Agammaglobulinemia (Bruton’s Disease)** is a primary immunodeficiency caused by a mutation in the **Bruton Tyrosine Kinase (BTK) gene**. This defect leads to a failure of pre-B cells to differentiate into mature B cells (CD19/CD20+). **Why Option A is Correct:** B-cells are the primary constituents of the germinal centers in peripheral lymphoid tissues. In XLA, there is a profound lack of mature B-cells, resulting in the **hypoplasia of lymphoid tissues**. Clinically, this manifests as the absence of tonsils and non-palpable lymph nodes, which is a classic diagnostic clue in a child presenting with recurrent pyogenic infections (e.g., *S. pneumoniae, H. influenzae*) after 6 months of age. **Analysis of Incorrect Options:** * **Option B (Female sex):** XLA follows an **X-linked recessive** inheritance pattern, meaning it occurs almost exclusively in **males**. * **Option C (High isohemagglutinin titers):** Isohemagglutinins (Anti-A, Anti-B) are naturally occurring IgM antibodies. Since XLA patients cannot produce immunoglobulins, these titers will be **absent or extremely low**. * **Option D (Low CD3 count):** CD3 is a marker for **T-cells**. In XLA, T-cell numbers and functions are typically normal; the defect is strictly limited to the B-cell lineage. **NEET-PG High-Yield Pearls:** * **Flow Cytometry:** Shows absent or <2% CD19+ B-cells. * **Infections:** Susceptible to encapsulated bacteria and certain viruses (notably **Enteroviruses** like Echovirus, which can cause chronic meningoencephalitis). * **Vaccine Contraindication:** Live viral vaccines (especially **OPV**) are contraindicated due to the risk of vaccine-associated paralytic poliomyelitis. * **Treatment:** Lifelong Intravenous Immunoglobulin (IVIG) replacement.

Question 670: Which animal is commonly used to demonstrate anaphylaxis in laboratory settings?

A. Rabbit
B. Adult mice
C. Monkey
D. Guinea Pig (Correct Answer)

Explanation: **Explanation:** **Guinea pigs** are the animal of choice for demonstrating **Systemic Anaphylaxis** (Type I Hypersensitivity) because they are exquisitely sensitive to histamine and anaphylactic triggers. 1. **Why Guinea Pigs?** They possess a unique physiological profile where the smooth muscles of the bronchioles are highly reactive. Upon secondary exposure to an antigen (the "challenging dose"), there is a massive release of pharmacological mediators (primarily histamine) from mast cells. This leads to rapid bronchoconstriction, respiratory distress, and asphyxiation, providing a clear, visible demonstration of anaphylactic shock. 2. **Why other options are incorrect:** * **Rabbit:** While used for producing antibodies (antisera) and the Pyrogen test, they are less sensitive to systemic histamine release compared to guinea pigs. * **Adult Mice:** Mice are relatively resistant to histamine. To induce anaphylaxis in mice, much higher doses of antigen or specific strains/potentiators are required, making them less ideal for standard demonstrations. * **Monkey:** While phylogenetically closer to humans, they are not used for routine laboratory demonstrations of anaphylaxis due to ethical concerns, cost, and a less predictable rapid-shock response compared to the guinea pig model. **High-Yield NEET-PG Pearls:** * **Prausnitz-Küstner (PK) Reaction:** Historically used to demonstrate Type I hypersensitivity by injecting serum from an allergic person into the skin of a non-allergic person. * **Schultz-Dale Phenomenon:** An *in vitro* demonstration of anaphylaxis using isolated smooth muscle (e.g., intestinal loop or uterus) from a sensitized guinea pig. * **Mediators:** Remember that while **Histamine** is the primary mediator in guinea pigs and humans, **Serotonin** is more significant in rats and mice.

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Immunology — MCQs

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