Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 961: A patient with systemic lupus erythematosus presents with recurrent infections. Laboratory tests reveal a low total hemolytic complement (CH50) level. What does this indicate about the patient's immune function?

A. Deficiency in complement pathway (Correct Answer)
B. Impaired phagocytosis
C. Decreased production of antibodies
D. Decreased T cell activity

Explanation: ***Deficiency in complement pathway*** - A **low CH50 (total hemolytic complement)** level indicates a functional defect in the **classical complement pathway** (C1-C9), hindering the immune system's ability to clear pathogens and immune complexes. - This deficiency can lead to an increased susceptibility to recurrent infections, particularly from **encapsulated bacteria** (e.g., *Streptococcus pneumoniae*, *Neisseria* species), as the complement system is vital for their opsonization and lysis. - In **SLE patients**, complement consumption due to immune complex formation commonly causes low CH50, contributing to both disease pathology and increased infection risk. *Decreased production of antibodies* - While recurrent infections can be associated with decreased antibody production (humoral immunity), a low CH50 specifically points to a problem with the **complement system**, not necessarily antibody synthesis. - Antibody levels (e.g., IgG, IgM, IgA) would need to be measured directly through serum immunoglobulin assays to assess this aspect of immune function. *Decreased T cell activity* - **T cell activity** is crucial for cell-mediated immunity and fighting intracellular pathogens, but a low CH50 primarily reflects a complement defect, not T cell dysfunction. - Assays like lymphocyte proliferation, CD4+/CD8+ T cell subset analysis, or delayed-type hypersensitivity testing would be required to evaluate T cell function. *Impaired phagocytosis* - While **phagocytosis** is the process by which immune cells engulf pathogens, a low CH50 points to a deficit in complement-mediated **opsonization** that facilitates phagocytosis, rather than intrinsic phagocytic cell dysfunction. - Complement deficiencies indirectly affect phagocytosis by reducing C3b opsonization, but CH50 is not a direct measure of phagocytic cell function itself (which would require tests like nitroblue tetrazolium or neutrophil oxidative burst assays).

Question 962: Which immunoglobulin primarily provides defense in the respiratory and gastrointestinal tracts?

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

Explanation: ***IgA*** - **IgA** is the primary immunoglobulin found in **mucosal secretions**, including those of the respiratory and gastrointestinal tracts. - It plays a crucial role in providing **local immunity** by preventing pathogen adherence to epithelial surfaces. *IgG* - **IgG** is the most abundant immunoglobulin in serum and provides **systemic immunity**, crossing the placenta to confer passive immunity to the fetus. - While present in some mucosal areas, it is not the **primary defense** in respiratory and gastrointestinal secretions. *IgM* - **IgM** is typically the **first antibody produced** during a primary immune response and is an effective agglutinator. - It mainly exists in the bloodstream, contributing to systemic immunity, and is not the predominant antibody in mucosal secretions. *IgD* - **IgD** is found on the surface of **B lymphocytes** and is involved in B cell activation and differentiation. - Its role in direct pathogen defense in the respiratory and gastrointestinal tracts is **minimal** compared to IgA.

Question 963: Identify the primary immune response triggered by the presence of Helicobacter pylori in the gastric mucosa.

A. Activation of complement system
B. Cytotoxic T cell activity
C. Antibody production
D. Induction of mucosal inflammation (Correct Answer)

Explanation: ***Induction of mucosal inflammation*** - The presence of *H. pylori* in the gastric mucosa directly triggers a robust **inflammatory response**, characterized by the infiltration of neutrophils, macrophages, and lymphocytes. - This **chronic inflammation** is the primary immune response and a key factor in the pathogenesis of *H. pylori*-associated diseases like gastritis, peptic ulcers, and gastric cancer. *Activation of complement system* - While complement activation can occur as part of the immune response, it is not the *primary* and most significant immediate response triggered by *H. pylori* in the mucosa. - *H. pylori* has evolved mechanisms to evade and resist complement-mediated killing, making it less effective as a primary defense. *Cytotoxic T cell activity* - **Cytotoxic T cells (CD8+)** are primarily involved in targeting and destroying virally infected cells or cancer cells, not typically extracellular bacterial pathogens like *H. pylori*. - While some T cell activity contributes to the overall immune response, it's not the initial or primary mechanism for directly combating *H. pylori* in the mucosa. *Antibody production* - **Antibody production** (humoral immunity) develops as a later, adaptive immune response to *H. pylori* infection. - While important for long-term immunity and diagnostics (e.g., serology), it is not the *primary* immediate immune response triggered upon initial detection in the gastric mucosa.

Question 964: Which type of antigen-antibody reaction is typically involved in the precipitation test?

A. Soluble antigen-antibody complex formation (Correct Answer)
B. Cell lysis assessment
C. Clumping of particles
D. Particulate antigen-antibody interaction

Explanation: ***Soluble antigen-antibody complex formation*** - Precipitation tests rely on the formation of **insoluble macromolecular complexes** when soluble antigens and antibodies are mixed in optimal proportions. - This complex then becomes visible as a **precipitate**, indicating the presence of specific antigens or antibodies. *Cell lysis assessment* - Cell lysis is characteristic of **complement-mediated reactions** or certain assays like the **complement fixation test**, not precipitation. - This involves the destruction of cells rather than the formation of visible antigen-antibody complexes. *Clumping of particles* - Clumping of particles is the hallmark of **agglutination reactions**, where antibodies bind to particulate antigens (e.g., on cells or latex beads), causing them to clump together. - Precipitation, in contrast, involves soluble antigens. *Particulate antigen-antibody interaction* - This describes **agglutination reactions**, where antigens are found on the surface of insoluble particles. - Precipitation reactions specifically involve **soluble antigens** forming an insoluble complex with antibodies.

Question 965: Which component of the complement system acts as an opsonin?

A. C3a
B. C3b (Correct Answer)
C. C5a
D. C6

Explanation: ***C3b*** - **C3b** is a crucial component of the complement system that acts as an **opsonin**, binding to microbial surfaces and C3b receptors on phagocytes. - Opsonization by C3b enhances **phagocytosis** by coating pathogens, making them more easily recognized and engulfed by immune cells. *C3a* - **C3a** is an **anaphylatoxin** that promotes inflammation by inducing mast cell degranulation and vasodilation. - It does not directly opsonize pathogens but plays a role in attracting immune cells to the site of infection. *C5a* - **C5a** is a potent **anaphylatoxin** and **chemoattractant** for neutrophils and monocytes, promoting inflammation. - While it contributes to the immune response, **C5a** itself does not act as an opsonin. *C6* - **C6** is a component of the **Membrane Attack Complex (MAC)**, which forms pores in the membrane of target cells, leading to lysis. - It is involved in pathogen killing but does not have opsonin activity.

Question 966: Whole blood is used as a sample for which of the following tests?

A. Genexpert
B. Blood Culture for Bacteria
C. Interferon Gamma Release Assay (IGRA) (Correct Answer)
D. Serological Test for Viruses

Explanation: ***Interferon Gamma Release Assay (IGRA)*** - IGRAs, such as QuantiFERON-TB Gold, directly measure **interferon-gamma release** from T-lymphocytes stimulated by *Mycobacterium tuberculosis* antigens. - This test requires **fresh whole blood** as the living lymphocytes are essential for the immune response measured. *Blood Culture for Bacteria* - While blood is cultured, the primary goal is to **isolate and identify viable bacteria** from the bloodstream, not to detect an immune response within the whole blood itself. - Blood cultures typically involve placing blood into specific **culture media** to promote bacterial growth. *Genexpert* - The GeneXpert MTB/RIF assay is a **molecular test** used to detect *Mycobacterium tuberculosis* DNA and rifampicin resistance. - This assay is typically performed on **sputum samples** or other bodily fluids, not whole blood, to diagnose active tuberculosis. *Serological Test for Viruses* - Serological tests for viruses detect **antibodies or antigens** in the blood, which circulate in the **plasma or serum** component of blood. - These tests typically use **separated serum or plasma**, rather than whole blood, as the cellular components are not required for antibody or antigen detection.

Question 967: Antibody specificity is due to ?

A. Amino acid sequence in the variable region of both chains (Correct Answer)
B. Amino acid sequence in the variable region of the light chain
C. Amino acid sequence in the variable region of the heavy chain
D. Amino acid at the carboxy terminal of the heavy chain

Explanation: ***Amino acid sequence in the variable region of both chains*** - Antibody specificity is determined by the **variable (V) regions of both heavy and light chains** working together to form the antigen-binding site (paratope). - The antigen-binding site contains **6 complementarity-determining regions (CDRs)**: 3 from the heavy chain (H1, H2, H3) and 3 from the light chain (L1, L2, L3). - **Both chains contribute equally** to antigen recognition - the CDRs from both chains create the three-dimensional binding pocket that determines specificity. - This is the fundamental principle of antibody diversity and specificity in adaptive immunity. *Amino acid sequence in the variable region of the heavy chain* - While the heavy chain variable region is essential, it **cannot determine specificity alone** without the light chain contribution. - The heavy chain CDRs (especially CDR-H3, the most variable region) contribute significantly, but specificity requires **both chains**. - This option is incomplete as it ignores the critical contribution of light chain CDRs. *Amino acid at the carboxy terminal of the heavy chain* - The carboxy-terminal (C-terminal) is part of the **constant region (Fc region)** of the heavy chain. - This region determines the antibody **isotype** (IgG, IgM, IgA, IgD, IgE) and mediates **effector functions** (complement fixation, Fc receptor binding). - The constant region does **not** determine antigen specificity. *Amino acid sequence in the variable region of the light chain* - The light chain variable region is crucial for antigen binding, but it **cannot determine specificity alone** without the heavy chain contribution. - Light chain CDRs (L1, L2, L3) form part of the antigen-binding site but require the heavy chain CDRs to create the complete binding pocket. - This option is incomplete as it ignores the critical contribution of heavy chain CDRs.

Question 968: What determines the specificity of an antibody?

A. Fc portion
B. Fab region (Correct Answer)
C. Carboxy terminal
D. None of the options

Explanation: ***Fab region*** - The **Fab (Fragment antigen-binding) region** contains the **variable domains** of both the heavy and light chains, which form the antigen-binding site. - This region is responsible for recognizing and binding to specific **epitopes** on antigens, thus determining the antibody's specificity. *Fc portion* - The **Fc (Fragment crystallizable) portion** primarily mediates effector functions such as binding to **Fc receptors** on immune cells and activating complement. - It does not directly participate in antigen recognition and therefore does not determine the antibody's specificity. *Carboxy terminal* - The **carboxy terminal** refers to the end of the polypeptide chain, specifically on the heavy chain within the Fc region, or at the end of the light chain. - While part of the antibody structure, it does not specifically determine the unique antigen-binding characteristics. *None of the options* - This option is incorrect because the **Fab region** is indeed responsible for determining antibody specificity.

Question 969: Which of the following mechanisms primarily contributes to antibody diversity?

A. Antigenic variation
B. Gene rearrangement (Correct Answer)
C. Gene translocation
D. a and c

Explanation: ***Gene rearrangement*** - **V(D)J recombination** (Variable, Diversity, Joining segments) is the primary mechanism that shuffles and combines different gene segments to create unique antibody genes. - This process occurs in **B lymphocytes** during their development, generating a vast repertoire of immunoglobulin specificities. *Gene translocation* - Refers to the movement of chromosomal segments to new locations, which can lead to **oncogene activation** or fusion proteins, but does not primarily contribute to antibody diversity in the same way as V(D)J recombination. - While translocations can be involved in some B-cell malignancies, they are not a normal physiological mechanism for generating antibody diversity. *Antigenic variation* - This mechanism is primarily utilized by **pathogens** to evade host immune responses by altering their surface antigens. - It does not contribute to the initial generation of antibody diversity within the host immune system but rather describes how pathogens change to avoid existing antibodies. *a and c* - This option is incorrect because while **gene rearrangement** (a) is a primary mechanism for antibody diversity, **antigenic variation** (c) is a mechanism used by pathogens to evade immunity and does not contribute to the generation of antibody diversity within the host.

Question 970: Reactive nitrogen species for killing of microbes are mainly derived from

A. Nitric Oxide [NO] (Correct Answer)
B. Elemental nitrogen [N2]
C. Nitrogen Dioxide [NO2]
D. Nitrous Oxide [N2O]

Explanation: ***Nitric Oxide [NO]*** - **Nitric oxide (NO)** is a key reactive nitrogen species produced by immune cells (e.g., macrophages) via nitric oxide synthase, which is crucial for killing intracellular pathogens. - It reacts with **superoxide radicals** to form other highly reactive and potent antimicrobial species like **peroxynitrite**. *Elemental nitrogen [N2]* - **Elemental nitrogen (N2)** is an inert gas constituting about 78% of the atmosphere and is not directly involved in the immune killing of microbes. - It must be "fixed" into biologically available forms by specific bacteria, a process distinct from host immune responses. *Nitrogen Dioxide [NO2]* - **Nitrogen dioxide (NO2)** can be formed from NO, particularly in the presence of oxygen, and can act as a reactive species, but **NO** is the primary precursor generated by immune cells for direct antimicrobial action. - While it has some oxidative potential, it is not the main biologically derived RNS for direct microbial control within phagocytes. *Nitrous Oxide [N2O]* - **Nitrous oxide (N2O)**, commonly known as laughing gas, is primarily used as an anesthetic agent and is not a significant **reactive nitrogen species** (RNS) involved in the host immune defense against microbes. - It is metabolically stable and does not participate in the cytotoxic reactions against pathogens like NO.

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Cells and Organs of Immune System

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Innate Immunity

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Immunology — MCQs

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