Immunology Practice Questions

Q: Which of the following statements is true about interferons?

Host protein. **Explanation:** Interferons (IFNs) are low-molecular-weight **host-encoded proteins** (specifically glycoproteins) produced by cells in response to viral infections or other stimuli. They are a critical component of the innate immune system. * **Why Option A is Correct:** Interferons are synthesized by the host cell's ribosomes using host genetic templates. When a cell is infected by a virus, it produces IFNs to signal neighboring cells to enter an "antiviral state." * **Why Option B is Incorrect:** They are not encoded by the viral genome. They are the host's defensive response to the presence of viral double-stranded RNA or other pathogen-associated molecular patterns (PAMPs). * **Why Option C is Incorrect:** Interferons are proteins, not nucleic acids. Therefore, they are **resistant to nucleases** (RNase and DNase) but are inactivated by proteolytic enzymes (proteases). * **Why Option D is Incorrect:** Interferons are **species-specific but NOT virus-specific**. This means human interferon will work against many different types of viruses (Influenza, Hepatitis, etc.) in human cells, but human interferon will not be effective in mouse cells. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** IFNs do not act directly on the virus. They induce the synthesis of host enzymes like **2',5'-oligoadenylate synthetase** and **protein kinase R (PKR)**, which inhibit viral protein synthesis and degrade viral mRNA. * **Classification:** * **Type I (IFN-α, IFN-β):** Produced by leucocytes and fibroblasts; primarily antiviral. * **Type II (IFN-γ):** Produced by T-cells and NK cells; primarily immunomodulatory (activates macrophages). * **Clinical Use:** Recombinant IFNs are used in treating Hepatitis B, Hepatitis C, Kaposi Sarcoma, and Multiple Sclerosis (IFN-β).

Q: Omenn syndrome represents which of the following immunological conditions?

Subset of T cell deficiency. **Explanation:** **Omenn Syndrome** is a rare, autosomal recessive form of **Severe Combined Immunodeficiency (SCID)**. It is primarily caused by hypomorphic (partial) mutations in the **RAG1 or RAG2 genes**. 1. **Why the correct answer is right:** In Omenn Syndrome, the RAG gene mutations lead to limited V(D)J recombination. This results in a profound deficiency of B cells, but a **paradoxical expansion of a few restricted, oligoclonal T-cell subsets**. These T cells are poorly regulated, autoreactive, and infiltrate peripheral tissues (skin, gut), leading to the characteristic clinical presentation of erythroderma and hepatosplenomegaly. Thus, it is classified as a specific subset of T-cell deficiency/dysfunction. 2. **Why the incorrect options are wrong:** * **HLA Class II Deficiency (Bare Lymphocyte Syndrome Type II):** Caused by mutations in transcription factors (like CIITA) required for HLA Class II expression. It presents with a lack of CD4+ T cells, not the oligoclonal T-cell expansion seen in Omenn. * **HLA Class I Deficiency (Bare Lymphocyte Syndrome Type I):** Usually due to TAP1/TAP2 mutations; it presents with chronic respiratory infections and skin ulcers, not SCID-like features. * **Purine Metabolism Deficiency:** Refers to **Adenosine Deaminase (ADA) deficiency**, which causes a complete absence of T, B, and NK cells (T-B-NK- SCID) due to toxic metabolite accumulation. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Erythroderma (peeling skin), Hepatosplenomegaly, and Lymphadenopathy. * **Laboratory Hallmarks:** Elevated **IgE** levels, **Eosinophilia**, and absent B cells. * **Genetics:** Most commonly **RAG1/RAG2** mutations (hypomorphic). * **Treatment:** Hematopoietic stem cell transplant (HSCT) is the definitive cure.

Q: Which of the following is a treatment for Type I hypersensitivity?

Sodium cromoglycate. ### Explanation **Correct Option: C. Sodium cromoglycate** Type I hypersensitivity (Immediate Hypersensitivity) is mediated by **IgE antibodies** binding to mast cells and basophils. Upon re-exposure to an allergen, cross-linking of these antibodies triggers **mast cell degranulation**, releasing inflammatory mediators like histamine. **Sodium cromoglycate** acts as a **mast cell stabilizer**. It prevents the influx of calcium across the mast cell membrane, thereby inhibiting degranulation and the release of histamine and leukotrienes. It is used prophylactically in conditions like bronchial asthma and allergic rhinitis. **Analysis of Incorrect Options:** * **A. Histamine:** This is the primary mediator *responsible* for the symptoms of Type I hypersensitivity (vasodilation, bronchoconstriction). Administering it would worsen the reaction. Antihistamines, not histamine, are used for treatment. * **B. IgA immunoglobulin:** IgA is the primary antibody in mucosal immunity. Type I hypersensitivity is specifically mediated by **IgE**. IgA has no therapeutic role in stopping an acute allergic response. * **D. Interleukin 5 (IL-5):** IL-5 is a cytokine that promotes **eosinophil** activation and chemotaxis. Inhibiting IL-5 (e.g., with Mepolizumab) can be a treatment strategy for severe eosinophilic asthma, but IL-5 itself would promote the allergic inflammatory cascade. **NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC):** For systemic anaphylaxis (Type I), the DOC is **Adrenaline (1:1000 IM)**. * **Mechanism:** Sodium cromoglycate does *not* antagonize histamine receptors; it prevents the release of histamine entirely. * **Type I Reaction Phases:** Initial phase (mast cells/histamine) occurs within minutes; Late-phase (eosinophils/leukotrienes) occurs 2–8 hours later. * **Other Mast Cell Stabilizers:** Nedocromil and Ketotifen.

Q: Which complement component is common to both the classical and alternative pathways?

C3. **Explanation:** The complement system is a biochemical cascade of the innate immune system consisting of three pathways: Classical, Alternative, and Lectin. **C3 (Option A)** is the correct answer because it represents the **central convergence point** of all complement pathways. - In the **Classical pathway**, the C3 convertase (C4b2a) cleaves C3 into C3a and C3b. - In the **Alternative pathway**, the C3 convertase (C3bBb) also cleaves C3. The activation of C3 is the most critical step in the cascade, leading to opsonization (via C3b) and the eventual formation of the Membrane Attack Complex (MAC). **Analysis of Incorrect Options:** * **C1q (Option C):** This is the recognition unit exclusive to the **Classical pathway** (triggered by IgG or IgM antigen-antibody complexes). It is not involved in the Alternative pathway. * **C5 (Option B):** While C5 is involved in both pathways, it is located *downstream* of the C3 cleavage. C3 is considered the "common component" where the initial amplification loops merge. * **C8 (Option D):** This is a component of the **Membrane Attack Complex (C5b-9)**. While it is part of the common terminal pathway, C3 is the primary functional link that initiates the late-phase components. **High-Yield Clinical Pearls for NEET-PG:** * **C3 deficiency:** The most severe complement deficiency, leading to recurrent pyogenic infections (e.g., *S. pneumoniae*) and Type III hypersensitivity reactions. * **C1, C2, C4 deficiency:** Strongly associated with **Systemic Lupus Erythematosus (SLE)**. * **C5-C9 (MAC) deficiency:** Specifically predisposes patients to recurrent **Neisserial infections** (Meningitis/Gonorrhea). * **Alternative Pathway Trigger:** Does not require antibodies; it is triggered directly by microbial surfaces (Endotoxins/LPS).

Q: Which of the following statements characterizes idiotypic determinants?

They can be antigenic.. ### Explanation **Concept Overview:** Immunoglobulins are classified based on three types of antigenic determinants: **Isotypes** (species-specific), **Allotypes** (individual-specific), and **Idiotypes** (clone-specific). **Why Option C is Correct:** An **Idiotype** refers to the unique set of antigenic determinants (idiotopes) located in the **variable regions** (V_H and V_L) of an antibody molecule. Specifically, these are found within the **antigen-binding site (paratope)**. Because these regions are unique to a specific clone of B-cells, the body can recognize them as "foreign" under certain conditions. Therefore, idiotypes **can be antigenic**, stimulating the production of **anti-idiotypic antibodies**. This interaction forms the basis of the "Jerne’s Idiotypic Network Theory" for immune regulation. **Why Other Options are Incorrect:** * **Option A & D:** Idiotypic determinants are located in the **variable regions** (Fab fragment), not the constant regions or the Fc (crystallizable) fragment. The constant regions define the **Isotype** (e.g., IgG vs. IgM). * **Option B:** Idiotypes are specific to **antibodies (immunoglobulins)** and T-cell receptors, not to general protein antigens. **High-Yield Clinical Pearls for NEET-PG:** * **Isotype:** Determined by heavy chain constant regions (Gamma, Alpha, Mu, Epsilon, Delta). * **Allotype:** Based on allelic polymorphism (e.g., Gm marker on IgG); differs between individuals of the same species. * **Idiotype:** Unique to a single antibody clone; determines antigen specificity. * **Clinical Application:** Anti-idiotypic antibodies are being researched as potential vaccines (e.g., in B-cell lymphomas) to trigger a specific immune response against malignant B-cell clones.

Q: C-reactive protein is:

Increased in pneumococcal infection. **Explanation:** **C-reactive protein (CRP)** is a classic **Acute Phase Reactant (APR)**, specifically a member of the pentraxin family. It is synthesized by the liver in response to pro-inflammatory cytokines, primarily **Interleukin-6 (IL-6)**. **Why Option D is Correct:** CRP was originally discovered because it reacts with the **C-polysaccharide** of *Streptococcus pneumoniae*. Its levels rise rapidly (within 6–48 hours) during acute inflammation or tissue injury. Therefore, it is significantly **increased in pneumococcal infections** and other bacterial infections, serving as a sensitive but non-specific marker of inflammation. **Why Other Options are Incorrect:** * **Option A:** CRP is **not an antibody**. While it can act as an opsonin and activate the classical complement pathway, it is an innate immune protein, not an immunoglobulin produced by B-cells. * **Option B:** CRP has no etiological link to **pneumoconiosis** (occupational lung diseases like silicosis). While it may be elevated due to chronic inflammation in such conditions, it is not "derived" from them. * **Option C:** Modern clinical laboratories detect CRP using **Latex Agglutination** or highly sensitive **Nephelometry/Turbidimetry**, rather than simple precipitation reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Kinetics:** CRP has a short half-life (~19 hours), making it an excellent marker for monitoring disease activity and response to antibiotic therapy. * **CRP vs. ESR:** CRP is a more sensitive and faster indicator of acute inflammation than the Erythrocyte Sedimentation Rate (ESR). * **hs-CRP:** High-sensitivity CRP is used as a biomarker to assess the risk of **Cardiovascular Disease (CVD)**. * **Universal Marker:** It is elevated in bacterial infections, Rheumatic fever, Rheumatoid Arthritis, and post-surgery, but typically remains low or mildly elevated in viral infections.

Q: Both antibody-dependent and independent complement pathways converge on which complement component?

C3. **Explanation:** The complement system consists of three distinct pathways: the **Classical** (antibody-dependent), the **Alternative** (antibody-independent), and the **Lectin** pathway. 1. **Why C3 is correct:** All three pathways converge at the formation of **C3 convertase**. This enzyme cleaves C3 into C3a (anaphylatoxin) and C3b (opsonin). C3 is the most abundant complement protein in the serum and represents the "final common pathway" for the initial activation steps. Once C3 is cleaved, the pathways merge into the terminal lytic sequence. 2. **Why other options are incorrect:** * **C1q:** This is the recognition unit specifically for the **Classical pathway** only. It binds to the Fc portion of IgM or IgG. It is not involved in the Alternative or Lectin pathways. * **C5:** While all pathways eventually lead to the activation of C5 to form the Membrane Attack Complex (MAC), the actual point of convergence where the distinct activation cascades meet is at the **C3 level**. C5 activation occurs downstream of C3. * **C8:** This is a component of the terminal Membrane Attack Complex (C5b-9). It is involved much later in the cascade, long after the pathways have converged. **High-Yield Clinical Pearls for NEET-PG:** * **C3 deficiency:** The most severe complement deficiency, leading to recurrent pyogenic infections (e.g., *S. pneumoniae*) and Type III hypersensitivity reactions. * **C1 esterase inhibitor deficiency:** Leads to **Hereditary Angioedema** (due to overproduction of bradykinin). * **C5-C9 deficiency:** Specifically predisposes patients to recurrent **Neisseria** infections (meningitis and gonorrhea). * **Opsonization:** C3b is the primary opsonin; **C3b** binds to **B**acteria (easy mnemonic).

Q: Which immunoglobulin acts as the B cell receptor?

IgM. **Explanation:** The B cell receptor (BCR) is a transmembrane protein complex located on the surface of B cells. It is composed of a membrane-bound immunoglobulin (mIg) molecule and a signal transduction moiety (Ig-α/Ig-β). **Why IgM is the correct answer:** Naïve B cells (B cells that have not yet encountered an antigen) predominantly express **monomeric IgM** and **IgD** on their surfaces. Among these, **IgM** is the primary functional receptor responsible for antigen recognition and the subsequent activation of the B cell. While IgM in the serum exists as a pentamer, it exists strictly as a monomer when acting as a BCR. **Analysis of Incorrect Options:** * **IgG:** This is the most abundant immunoglobulin in the serum and is responsible for secondary immune responses, opsonization, and placental transfer. It is not found on naïve B cells. * **IgA:** Primarily found in secretions (mucosal immunity) as a dimer. It does not function as a primary BCR on naïve cells. * **IgD:** Although IgD is co-expressed with IgM on naïve B cells, its specific physiological function is less defined, and IgM is considered the classic marker and functional receptor for B cell activation. **NEET-PG High-Yield Pearls:** * **Isotype Switching:** Once a B cell is activated, it can switch from producing IgM to IgG, IgA, or IgE; however, the specificity for the antigen remains the same. * **Pentameric vs. Monomeric:** Serum IgM is a **pentamer** (connected by a J-chain) and is the first antibody produced in a primary immune response. Surface IgM (BCR) is always a **monomer**. * **B Cell Markers:** Apart from the BCR, B cells are identified by surface markers **CD19, CD20, and CD21**.

Q: Which of the following immunoglobulins shows homocytotropism?

IgE. ### Explanation **Correct Answer: C. IgE** **Concept of Homocytotropism:** Homocytotropism refers to the unique property of an antibody to bind specifically to the surface of cells (primarily mast cells and basophils) of the same species. This binding occurs via the **Fc region** of the antibody to high-affinity **FcεRI receptors** on the target cells. **IgE** is the classic homocytotropic antibody. When an allergen cross-links these cell-bound IgE molecules, it triggers degranulation and the release of pharmacological mediators like histamine, leading to Type I Hypersensitivity reactions. **Analysis of Incorrect Options:** * **IgG (Option A):** While IgG is the most abundant antibody and can cross the placenta, it is generally not homocytotropic. Some subclasses (like IgG4) may show weak binding, but IgE is the definitive answer for this property. * **IgA (Option B):** Known as the "secretory antibody," its primary role is mucosal immunity. It exists as a dimer in secretions and does not bind to mast cells to mediate anaphylaxis. * **IgD (Option D):** Primarily acts as a B-cell surface receptor. Its precise systemic function is less defined, and it does not exhibit homocytotropic properties. **High-Yield NEET-PG Pearls:** * **Prausnitz-Küstner (PK) Reaction:** A classic historical test used to demonstrate homocytotropism by injecting serum from an allergic individual into the skin of a non-allergic person. * **Heat Lability:** IgE is the most heat-labile immunoglobulin (inactivated at 56°C for 30–60 minutes). * **Reaginic Antibody:** IgE is also known as the reagin antibody. * **Parasitic Infections:** IgE levels are characteristically elevated in helminthic infections and Type I hypersensitivity (Atopy).

Question 1

Which of the following statements is true about interferons?

Accepted Answer

Host protein

Answer

Viral protein

Answer

Inactivated by nuclease

Answer

Virus specific

Question 2

Omenn syndrome represents which of the following immunological conditions?

Accepted Answer

Subset of T cell deficiency

Answer

Human Leukocyte antigen (HLA) Class II Deficiency

Answer

HLA Class I Deficiency

Answer

Purine Metabolism Deficiency

Question 3

Which of the following is a treatment for Type I hypersensitivity?

Accepted Answer

Sodium cromoglycate

Answer

Histamine

Answer

IgA immunoglobulin

Answer

Interleukin 5

Question 4

Which complement component is common to both the classical and alternative pathways?

Accepted Answer

C3

Answer

C5

Answer

Clq

Answer

C8

Question 5

Which of the following statements characterizes idiotypic determinants?

Accepted Answer

They can be antigenic.

Answer

They are found in the crystallizable fragment (Fc) of immunoglobulins.

Answer

They are found on protein antigens.

Answer

They are found in the constant regions of immunoglobulins.

Question 6

C-reactive protein is:

Accepted Answer

Increased in pneumococcal infection

Answer

An antibody produced as a result of pneumococcal infection

Answer

Derived from pneumoconiosis

Answer

Detected by precipitation reaction

Question 7

Both antibody-dependent and independent complement pathways converge on which complement component?

Accepted Answer

C3

Answer

C5

Answer

Clq

Answer

C8

Question 8

An IgG2 molecule is composed of which of the following?

Accepted Answer

Two gamma2 chains and two kappa chains

Answer

One alpha, one gamma, and two kappa chains

Answer

One gamma1 chain and two kappa chains

Answer

Two gamma1 chains and one kappa and one lambda chain

Question 9

Which immunoglobulin acts as the B cell receptor?

Accepted Answer

IgM

Answer

IgG

Answer

IgA

Answer

IgD

Question 10

Which of the following immunoglobulins shows homocytotropism?

Accepted Answer

IgE

Answer

IgG

Answer

IgA

Answer

IgD

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?