Immunology Practice Questions

Q: Immunoglobulin variation does not depend on which of the following?

Light chain. ### Explanation The classification and variation of immunoglobulins are fundamentally determined by the **Heavy (H) chain**, not the light chain. **1. Why "Light chain" is the correct answer:** Immunoglobulin classes (Isotypes) such as IgG, IgA, IgM, IgD, and IgE are defined solely by the antigenic differences in their **Heavy chains** (γ, α, μ, δ, and ε respectively). While there are two types of light chains (Kappa and Lambda), they are present in all five classes of immunoglobulins. Therefore, the specific variation or "class" of an antibody does **not** depend on which light chain is present. **2. Analysis of Incorrect Options:** * **Heavy chain:** This is the primary determinant of the immunoglobulin class and its effector functions (e.g., placental transfer, complement activation). * **Amino acid sequence:** Variations in the amino acid sequence in the constant region determine the **Isotype**, while variations in the variable region determine the **Idiotype** (antigen-binding specificity). Thus, variation is highly dependent on sequence. * **Constant region:** The constant region of the heavy chain determines the biological properties and the class of the antibody. Isotypic variation is specifically located in the constant region. ### High-Yield Clinical Pearls for NEET-PG: * **Isotype:** Determined by the Heavy chain constant region (defines IgG, M, A, D, E). * **Allotype:** Determined by allelic differences at specific loci (e.g., Gm on heavy chain, Km on light chain). * **Idiotype:** Determined by the hypervariable region (antigen-binding site). * **Ratio:** In humans, the normal Kappa (κ) to Lambda (λ) ratio is **2:1**. A significant shift in this ratio (e.g., 10:1) suggests monoclonal proliferation, such as **Multiple Myeloma**. * **Valency:** IgG is bivalent, while secreted IgA is tetravalent (dimer) and IgM is decavalent (pentamer).

Q: Which protein is responsible for the clearance of C3 convertase?

CD 55. **Explanation:** The correct answer is **CD 55**, also known as **Decay-Accelerating Factor (DAF)**. **Why CD 55 is correct:** CD 55 is a membrane-bound protein found on the surface of blood cells and endothelial cells. Its primary function is to protect host cells from accidental damage by the complement system. It acts by binding to the **C3 convertase** (C4b2a in the classical pathway and C3bBb in the alternative pathway) and accelerating its dissociation (decay). By clearing C3 convertase, it prevents the amplification of the complement cascade and the subsequent formation of the Membrane Attack Complex (MAC). **Analysis of Incorrect Options:** * **CD 59 (Protectin):** This protein acts further downstream in the cascade. It prevents the final assembly of the **Membrane Attack Complex (MAC)** by inhibiting the binding of C9 to the C5b-8 complex. * **Factor D:** This is a serine protease essential for the **activation** of the alternative pathway (it cleaves Factor B into Bb). It promotes, rather than clears, C3 convertase formation. * **Factor E:** This is a distractor; there is no "Factor E" in the human complement system. **Clinical Pearls for NEET-PG:** * **Paroxysmal Nocturnal Hemoglobinuria (PNH):** This condition is caused by a deficiency of **PIGA**, a protein required to anchor CD 55 and CD 59 to the cell membrane via GPI anchors. Without these regulators, RBCs are susceptible to complement-mediated lysis, leading to hemolytic anemia. * **C3 Convertases:** Remember C4b2a (Classical/Lectin) and C3bBb (Alternative). * **Factor I and Factor H:** These are other important regulators; Factor I cleaves C3b and C4b, while Factor H is the main soluble regulator of the alternative pathway.

Q: A 4-year-old child presents with recurrent pyogenic infections caused by organisms with polysaccharide capsules. Which of the following immunoglobulin deficiencies should be investigated?

IgG 2. **Explanation:** The correct answer is **C. IgG 2**. **Medical Concept:** IgG is divided into four subclasses (IgG1, IgG2, IgG3, and IgG4). Each subclass has a specific affinity for different types of antigens. **IgG2** is the primary subclass responsible for the immune response against **polysaccharide antigens**, which are found in the capsules of bacteria such as *Streptococcus pneumoniae*, *Haemophilus influenzae* type b, and *Neisseria meningitidis*. In children, the ability to produce IgG2 matures slowly (reaching adult levels around age 10), making them more susceptible to these encapsulated organisms. A selective deficiency in IgG2 leads to recurrent sinopulmonary infections despite normal total IgG levels. **Analysis of Options:** * **IgA (Option A):** While IgA deficiency is the most common primary immunodeficiency, it typically presents with mucosal infections or is asymptomatic. It is not specifically linked to the failure of polysaccharide-encapsulated bacterial clearance. * **IgG1 (Option B):** IgG1 is the most abundant subclass and primarily responds to **protein antigens** (e.g., toxins like tetanus/diphtheria). * **IgA + IgG2 (Option D):** While IgG2 deficiency often coexists with IgA deficiency, the question specifically asks which deficiency is responsible for the failure to handle **polysaccharide capsules**. IgG2 is the specific mediator for this response. **NEET-PG High-Yield Pearls:** * **IgG1 & IgG3:** Respond to protein antigens (viral proteins). * **IgG2 & IgG4:** Respond to carbohydrate/polysaccharide antigens. * **Clinical Hint:** If a child has recurrent pneumonia/otitis but a normal total IgG level, always check **IgG subclasses**. * **Wiskott-Aldrich Syndrome:** Often associated with low levels of antibodies to polysaccharide antigens.

Q: Which component of the complement system has cell lytic property?

Membrane Attack Complex (MAC). **Explanation:** The complement system is a biochemical cascade of the innate immune system that helps clear pathogens. The correct answer is **Membrane Attack Complex (MAC)** because it is the final effector unit of the complement pathways (Classical, Alternative, and Lectin) responsible for direct cell lysis. * **Why Option A is Correct:** The MAC is composed of complement components **C5b, C6, C7, C8, and multiple C9 molecules (C5b-9)**. It functions by inserting itself into the lipid bilayer of the target cell membrane, forming a transmembrane channel (pore). This disrupts the osmotic balance, leading to an influx of water and ions, which results in **osmotic lysis** of the cell. * **Why Options B, C, and D are Incorrect:** * **C3b:** Acts primarily as an **opsonin**. It coats bacteria to enhance phagocytosis by macrophages and neutrophils (via CR1 receptors). * **C3a and C5a:** These are known as **Anaphylatoxins**. They trigger mast cell degranulation, leading to histamine release, increased vascular permeability, and smooth muscle contraction. **C5a** is also a potent chemoattractant for neutrophils. **High-Yield Clinical Pearls for NEET-PG:** * **Deficiency:** Patients with deficiencies in late complement components (**C5 to C9**) cannot form the MAC and are uniquely susceptible to recurrent infections by ***Neisseria meningitidis*** and ***Neisseria gonorrhoeae***. * **Regulation:** **CD59 (Protectin)** is a host cell protein that inhibits MAC formation on self-cells; its deficiency leads to Paroxysmal Nocturnal Hemoglobinuria (PNH). * **Mnemonic:** "C3a, C4a, C5a cause **A**naphylaxis; C3b helps **B**acteria **B**ind (Opsonization)."

Q: What is true about the secondary immune response?

It is usually of low titre.. The secondary immune response (anamnestic response) occurs when the immune system encounters an antigen for the second or subsequent time. This response is mediated by **memory B and T cells** generated during the primary exposure. ### **Explanation of Options** * **Correct Answer (B):** While the question asks for what is "true," there appears to be a technical discrepancy in the provided key. In standard immunology, the secondary response is characterized by a **high titre** of antibodies. However, if this is a "Select the False statement" style question or refers to specific experimental conditions, the key indicates "low titre." *Note: In standard NEET-PG patterns, the secondary response is high titre, rapid, and long-lasting.* * **Option A (Incorrect):** The secondary response has a **short latent (lag) period** (typically 1–3 days) compared to the primary response (5–10 days) because memory cells are already primed. * **Option C (Incorrect):** Antibodies appear very **rapidly** and reach peak levels much faster than in the primary response. * **Option D (Incorrect):** The secondary response **persists for a much longer period**, providing prolonged immunity. ### **High-Yield NEET-PG Pearls** * **Antibody Class:** The primary response is dominated by **IgM**, whereas the secondary response is dominated by **IgG** (due to class switching). * **Affinity Maturation:** Antibodies in the secondary response have a **higher affinity** for the antigen compared to the primary response. * **Clinical Application:** This principle is the basis for **booster doses** in vaccination schedules to ensure high, long-lasting antibody titres. * **Memory Cells:** These cells do not require further differentiation to respond, which accounts for the lack of a significant lag phase.

Q: What is true about type II hypersensitivity reactions?

May be complement mediated. **Explanation:** Type II hypersensitivity, also known as **Cytotoxic Hypersensitivity**, occurs when IgG or IgM antibodies bind to antigens on the surface of specific cells or tissues. **Why Option A is Correct:** The binding of IgG or IgM to the cell surface activates the **Classical Complement Pathway**. This leads to the formation of the Membrane Attack Complex (MAC), causing direct cell lysis. Additionally, complement fragments like C3b act as opsonins, facilitating phagocytosis by macrophages. Examples include Autoimmune Hemolytic Anemia and Rh incompatibility. **Why Incorrect Options are Wrong:** * **Option B:** The **Schultz-Dale phenomenon** is a laboratory demonstration of smooth muscle contraction in response to a specific antigen in a sensitized animal; it is a classic feature of **Type I (Immediate) Hypersensitivity**, not Type II. * **Option C:** Type II reactions are strictly **Antibody-Dependent** (IgG and IgM). Antibody-independent reactions are characteristic of Type IV (Cell-mediated) hypersensitivity. * **Option D:** **IgE** is the primary mediator of Type I hypersensitivity (Allergy/Anaphylaxis). Type II involves IgG and IgM. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanisms of Type II:** 1. Complement-mediated lysis, 2. Antibody-dependent cell-mediated cytotoxicity (ADCC) via NK cells, 3. Anti-receptor activity (e.g., Myasthenia Gravis, Graves' disease). * **Direct Coombs Test:** Used to detect Type II reactions on RBC surfaces. * **Mnemonic:** Remember **ACID** (Type I: **A**naphylactic; Type II: **C**ytotoxic; Type III: **I**mmune Complex; Type IV: **D**elayed).

Q: Which category of immune disorders best describes Chediak-Higashi syndrome?

Disorders of phagocytosis. **Explanation:** **Chediak-Higashi Syndrome (CHS)** is a rare autosomal recessive disorder caused by a mutation in the **LYST gene** (Lysosomal Trafficking Regulator). This defect leads to impaired microtubule assembly, which prevents the fusion of phagosomes with lysosomes. Consequently, phagocytes (neutrophils and macrophages) can ingest bacteria but cannot kill them effectively because the **phagolysosome** fails to form. This places CHS firmly in the category of **Disorders of Phagocytosis**. **Analysis of Options:** * **Option A (Specific Immunity):** These involve defects in B-cells (antibody production) or T-cells (cell-mediated immunity), such as SCID or X-linked Agammaglobulinemia. CHS primarily affects the innate cellular response. * **Option B (Complement):** These involve deficiencies in proteins like C3 (recurrent infections) or C5-C9 (Neisserial infections). CHS is a cellular defect, not a humoral protein deficiency. * **Option D (Secondary Immunodeficiencies):** These are acquired due to external factors like HIV, malnutrition, or chemotherapy. CHS is a primary (congenital/genetic) immunodeficiency. **Clinical Pearls for NEET-PG:** 1. **Pathognomonic Finding:** Presence of **giant intracytoplasmic granules** in neutrophils and platelets on a peripheral blood smear. 2. **Clinical Triad:** * **Partial Oculocutaneous Albinism** (melanocyte transport defect). * **Recurrent Pyogenic Infections** (Staph and Strep). * **Neurological abnormalities** (peripheral neuropathy). 3. **Bleeding Diathesis:** Due to defective dense granules in platelets. 4. **Accelerated Phase:** A life-threatening lymphohistiocytic infiltration of organs (HLH-like syndrome).

Q: Which of the following conditions is characterized by normal or slightly reduced IgG levels?

Wiskott-Aldrich syndrome. **Explanation:** **Wiskott-Aldrich Syndrome (WAS)** is an X-linked recessive disorder caused by a mutation in the *WASP* gene, leading to defective actin polymerization in hematopoietic cells. The characteristic immunoglobulin profile in WAS is unique: **IgM levels are low**, while **IgA and IgE levels are elevated**. Crucially, **IgG levels are typically normal or only slightly reduced**, making it the correct answer. **Analysis of Incorrect Options:** * **Selective IgA Deficiency:** This is the most common primary immunodeficiency. It is characterized by a profound deficiency of IgA (<7 mg/dL) with **normal levels of IgG and IgM**. * **DiGeorge Syndrome:** This is a T-cell deficiency caused by the failure of the 3rd and 4th pharyngeal pouches to develop (22q11 deletion). While B-cell numbers are normal, antibody production (including IgG) may be impaired due to a lack of T-cell help, but the primary defect is cellular, not a specific "normal to slightly reduced" IgG pattern. * **Common Variable Immunodeficiency (CVID):** This condition is characterized by a **significant decrease in IgG**, along with low IgA and/or IgM. The hallmark of CVID is the failure of B-cells to differentiate into plasma cells. **High-Yield Clinical Pearls for NEET-PG:** * **WAS Triad:** Thrombocytopenia (with small platelets), Eczema, and Recurrent infections (primarily encapsulated organisms). * **Mnemonic for WAS:** **TIE** (Thrombocytopenia, Infections, Eczema). * **Laboratory Marker:** Low IgM + High IgA/IgE + Normal/Low IgG. * **Complications:** Increased risk of autoimmune diseases and B-cell lymphomas.

Question 1

Immunoglobulin variation does not depend on which of the following?

Accepted Answer

Light chain

Answer

Heavy chain

Answer

Amino acid sequence

Answer

Constant region

Question 2

Which protein is responsible for the clearance of C3 convertase?

Accepted Answer

CD 55

Answer

CD 59

Answer

Factor D

Answer

Factor E

Question 3

A 4-year-old child presents with recurrent pyogenic infections caused by organisms with polysaccharide capsules. Which of the following immunoglobulin deficiencies should be investigated?

Accepted Answer

IgG 2

Answer

IgA

Answer

IgG 1

Answer

IgA + IgG 2

Question 4

Which component of the complement system has cell lytic property?

Accepted Answer

Membrane Attack Complex (MAC)

Answer

C3b

Answer

C3a

Answer

C5a

Question 5

What is true about the secondary immune response?

Accepted Answer

It is usually of low titre.

Answer

It has a long latent period.

Answer

Antibodies appear in a short time.

Answer

It persists for a long time.

Question 6

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

Accepted Answer

Pneumocystis jirovecii pneumonia

Answer

Giardiasis

Answer

Recurrent sinusitis

Answer

Recurrent subcutaneous abscesses

Question 7

Skin transplant was done from sister to brother. After a few years, a skin transplant from brother to sister was done, but rejection occurred. This phenomenon is known as?

Accepted Answer

Eichwald-Silmser effect

Answer

Schultz-Dale phenomenon

Answer

Theobald Smith phenomenon

Answer

Shwartzman reaction

Question 8

What is true about type II hypersensitivity reactions?

Accepted Answer

May be complement mediated

Answer

The Schultz-Dale phenomenon is a type II hypersensitivity reaction

Answer

Antibody independent

Answer

Involves IgE

Question 9

Which category of immune disorders best describes Chediak-Higashi syndrome?

Accepted Answer

Disorders of phagocytosis

Answer

Disorders of specific immunity

Answer

Disorders of complement

Answer

Secondary immunodeficiencies

Question 10

Which of the following conditions is characterized by normal or slightly reduced IgG levels?

Accepted Answer

Wiskott-Aldrich syndrome

Answer

Selective IgA deficiency

Answer

DiGeorge syndrome

Answer

Common variable immunodeficiency

Immunology — MCQs

Immunology — MCQs

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