Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 1021: Which part of bacteria is most antigenic?

A. Lipopolysaccharide (Correct Answer)
B. Lipids
C. Nucleic acid
D. Protein coat

Explanation: ***Lipopolysaccharide (LPS)*** - The **O antigen** (polysaccharide component of LPS) in Gram-negative bacteria is one of the **most antigenic** bacterial components - Highly **immunogenic**, inducing strong antibody responses (both IgM and IgG) - Used as the basis for **serological typing** of Gram-negative bacteria (e.g., E. coli O157:H7) - The polysaccharide chains are structurally diverse with multiple epitopes, creating strain-specific immunity - While lipid A component has endotoxin activity, the polysaccharide portion is the primary antigenic determinant *Protein coat* - Bacterial **surface proteins** (flagella, pili, outer membrane proteins) are indeed antigenic - However, **polysaccharides** (including capsular polysaccharides and LPS) are classically considered more potent antigens - The term "protein coat" is also somewhat non-specific in bacteriology *Nucleic acid* - **Nucleic acids** (DNA, RNA) are generally **poor antigens** on their own - Not readily accessible to antibodies as they are intracellular - Can act as pathogen-associated molecular patterns (PAMPs) for innate immunity via TLRs, but are not major antibody targets *Lipids* - **Lipids** alone are generally **non-immunogenic** due to lack of structural complexity - Too small and lack sufficient epitopes to stimulate B cell responses effectively - May act as **haptens** requiring conjugation to carrier proteins

Question 1022: Opsonization takes place through -

A. C3a (anaphylatoxin)
B. C5a (anaphylatoxin)
C. C3b (Correct Answer)
D. C5b (initiates MAC formation)

Explanation: ***C3b*** - **C3b** is a key complement component that **binds to microbial surfaces**, marking them for phagocytosis by immune cells. - Phagocytic cells, such as macrophages and neutrophils, have **receptors for C3b**, facilitating the engulfment and destruction of pathogens. *C3a (anaphylatoxin)* - **C3a** is an **anaphylatoxin** that mediates inflammation by causing mast cell degranulation and smooth muscle contraction. - It does not directly participate in **opsonization**, which is the process of marking pathogens for phagocytosis. *C5a (anaphylatoxin)* - **C5a** is a potent **anaphylatoxin** and **chemoattractant** for neutrophils and macrophages, promoting inflammation. - While it recruits phagocytes, it does not directly coat pathogens to enhance their uptake, which is the role of opsonins. *C5b (initiates MAC formation)* - **C5b** initiates the formation of the **membrane attack complex (MAC)** by assembling with other complement components (C6, C7, C8, C9). - The MAC creates pores in pathogen membranes, leading to **cell lysis**, but C5b itself does not function as an opsonin.

Question 1023: Which component is considered the central part of the complement pathway?

A. C1 (complement component 1)
B. C3 (complement component 3) (Correct Answer)
C. C2 (complement component 2)
D. C5 (complement component 5)

Explanation: ***C3*** - **C3** is considered the central component because all three major pathways of complement activation (classical, alternative, and lectin) converge at the point of **C3 activation**. - Its cleavage product, **C3b**, is crucial for opsonization, formation of the C5 convertase, and initiating the assembly of the **membrane attack complex (MAC)**. *C1 (complement component 1)* - **C1** is the initial component of the **classical complement pathway** but does not play a direct role in the alternative or lectin pathways. - Its primary function is to bind to **antibody-antigen complexes** or directly to pathogens to activate C4 and C2. *C2 (complement component 2)* - **C2** is a component of the **classical** and **lectin pathways**, acting as a substrate for C1s and MASP to form the C3 convertase. - It is not involved in the initial activation of the **alternative pathway**. *C5 (complement component 5)* - **C5** is activated downstream of C3 and is a key component in the formation of the **membrane attack complex (MAC)**. - While critical for pathogen lysis, its activation is dependent on the prior cleavage of **C3** into C3b.

Question 1024: Which of the following statements is true regarding T cell independent antigens?

A. They primarily activate T-cells.
B. They primarily activate B-cells. (Correct Answer)
C. They primarily activate macrophages.
D. They primarily activate CD8+ T cells.

Explanation: ***Correct: They primarily activate B-cells*** - T-cell independent antigens are typically **polysaccharides** (TI-2) or **lipopolysaccharides** (TI-1) with repeating epitopes that can directly cross-link B cell receptors (BCRs) - This direct binding and cross-linking provide a strong enough signal to activate B cells and induce **antibody production** (mainly IgM) without the need for T cell help - They induce a rapid but limited immune response with minimal memory formation *Incorrect: They primarily activate T-cells* - T-cell independent antigens do not require processing and presentation by **MHC molecules**, which is essential for T cell activation - T cells recognize processed peptides presented by MHC, a mechanism not utilized by T-cell independent antigens - By definition, these antigens activate B cells **without** T cell involvement *Incorrect: They primarily activate macrophages* - While macrophages are antigen-presenting cells, their primary role in adaptive immunity is to process and present antigens to T cells - Macrophages are involved in **phagocytosis** and antigen processing, but are not the primary target cells for T-independent antigens - The key feature of TI antigens is direct B cell activation, not macrophage activation *Incorrect: They primarily activate CD8+ T cells* - **CD8+ T cells** are activated by processed antigens presented on **MHC class I molecules**, typically derived from intracellular pathogens - T-cell independent antigens do not utilize this pathway and are primarily involved in **humoral immunity** through direct B cell activation - TI antigens cannot activate CD8+ T cells as they bypass the T cell-dependent pathway entirely

Question 1025: When is the prozone phenomenon seen?

A. Same concentration of antibody and antigen
B. Hyperimmune reaction
C. In antigen excess to antibody
D. Antibody excess to antigen (Correct Answer)

Explanation: ***Antibody excess to antigen*** - The **prozone phenomenon** occurs when there is a significant **excess of antibodies** relative to the antigen, leading to inhibition of lattice formation. - In this state, too many antibodies bind to individual antigen sites, preventing cross-linking and thus inhibiting visible **agglutination** or **precipitation**. *Same concentration of antibody and antigen* - This scenario typically represents the **zone of equivalence**, where optimal lattice formation and visible reaction (agglutination or precipitation) occur. - It is where the concentrations of antibody and antigen are balanced, leading to maximum complex formation. *In antigen excess to antibody* - This situation is known as the **postzone phenomenon**, where an excess of antigen prevents the formation of stable antibody-antigen complexes. - The antigen saturates the limited antibody sites, resulting in no or minimal visible reaction. *Hyperimmune reaction* - A hyperimmune reaction refers to an **exaggerated immune response**, often resulting from repeated exposure to an antigen. - While it involves high antibody levels, it is a clinical state rather than a specific phenomenon describing antibody-antigen ratios in *in vitro* tests.

Question 1026: Which immunoglobulin is the most efficient at activating the classical complement pathway?

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

Explanation: ***IgM*** - **IgM** is a **pentamer** with **ten antigen-binding sites**, allowing it to bind multiple antigens on a surface - This **multivalency** enables formation of stable antigen-antibody complexes, creating an efficient platform for **C1q binding** - A **single IgM molecule** bound to antigen provides sufficient binding sites for activating C1q, making it the **most efficient** activator of the classical complement pathway - Its pentameric structure means it requires only **one molecule** to initiate complement activation *IgG* - IgG is a **monomer** and the **most abundant** serum immunoglobulin - Requires **at least two IgG molecules in close proximity** on an antigen surface to effectively activate the classical complement pathway - While it **can activate** the classical pathway, it is **less efficient per molecule** than pentameric IgM due to lower avidity for C1q *IgA* - Primarily functions in **mucosal immunity** (secretory IgA in saliva, tears, respiratory and GI tract) - **Poor activator** of the classical complement pathway - Can activate the **alternative pathway** of complement, but minimal role in classical pathway compared to IgM and IgG *IgD* - Primarily found on the **surface of naïve B lymphocytes** as a B cell receptor - Limited role as a **secreted antibody** in serum - **Does not activate** the complement pathway (neither classical nor alternative)

Question 1027: Which complement proteins are formed in the liver?

A. C3, C6
B. C2, C4 (Correct Answer)
C. C5, C8
D. C1

Explanation: ***C2, C4*** - While **all complement proteins (C1-C9) are primarily synthesized in the liver**, this question (NEET PG 2012) expects this as the answer based on the context of **classical pathway activation**. - **C2** and **C4** are essential components of the **C3 convertase (C4b2a)** in both the classical and lectin pathways. - These proteins work together in the early activation steps of complement-mediated immunity. - **Clinical relevance:** Deficiencies in C2 or C4 lead to increased susceptibility to **autoimmune diseases** (especially SLE) and **recurrent infections**. *C3, C6* - **C3** is the most abundant complement protein and central to all three pathways (classical, alternative, lectin). - **C6** is part of the membrane attack complex (MAC: C5b-C6-C7-C8-C9). - Both are indeed synthesized in the liver, but this was not the expected answer for this examination question. *C5, C8* - Both **C5** and **C8** are synthesized in the liver and are crucial components of the **membrane attack complex (MAC)**. - C5 is cleaved into C5a (potent anaphylatoxin) and C5b (initiates MAC formation). - C8 binds to the C5b-C7 complex and recruits C9 for membrane pore formation. *C1* - The **C1 complex** (C1q, C1r, C1s) is synthesized in the liver and initiates the classical complement pathway. - C1q recognizes antibody-antigen complexes (IgG or IgM bound to antigen). - **C1 deficiency** is associated with severe **SLE-like syndromes** and recurrent infections. **Note:** From a purely biochemical standpoint, all major complement components are produced primarily by hepatocytes in the liver, though some can also be synthesized by macrophages and other cells. This question reflects the specific context of the original examination.

Question 1028: Which of the following is the most potent stimulator of Naive T-cells?

A. Macrophages
B. B-cell
C. Mature dendritic cells (Correct Answer)
D. Follicular dendritic cells

Explanation: ***Mature dendritic cells*** - **Mature dendritic cells** are the most potent professional antigen-presenting cells (APCs) for activating **naive T cells** due to their efficient antigen processing, presentation abilities, and high expression of costimulatory molecules (e.g., CD80, CD86) and MHC-peptide complexes. - Activated by pathogens or inflammatory signals, they migrate to secondary lymphoid organs where they initiate primary immune responses by presenting antigens to and activating naive T cells. *Follicular dendritic cells* - **Follicular dendritic cells** primarily present intact antigens to **B cells** in germinal centers of secondary lymphoid organs, playing a crucial role in B cell maturation, selection, and antibody production. - They lack MHC class II molecules and thus cannot directly present antigens to naive T cells. *Macrophages* - While **macrophages** are professional APCs, they are generally less efficient than mature dendritic cells at activating **naive T cells**, especially in the initiation of primary immune responses. - They are more involved in presenting antigens to already activated T cells and clearing pathogens, often acting as secondary APCs. *B-cell* - **B cells** can act as APCs, but they are generally less efficient than **dendritic cells** in activating **naive T cells**, especially for the primary immune response. - Their primary role in antigen presentation is to present processed antigens to **helper T cells** to receive costimulation for their own activation and differentiation into plasma cells, often after being activated themselves.

Question 1029: HIV window period indicates:

A. time period between infection and onset of first symptoms
B. time period between infection and minimum viral load
C. time period between infection and maximum viral load
D. time period between infection and detection of antibodies (Correct Answer)

Explanation: ***time period between infection and detection antibodies*** - The **HIV window period** refers specifically to the time frame after initial infection during which **HIV antibodies** have not yet reached detectable levels in routine serological tests. - During this period, an infected individual can still transmit the virus, even if their test results would appear **negative** for antibodies. *time period between infection and onset of first symptoms* - The onset of **first symptoms**, such as acute retroviral syndrome, can occur before or after antibodies are detectable, and is not the defining characteristic of the window period. - Symptoms are a **clinical manifestation**, whereas the window period is a **diagnostic concept** related to test detectability. *time period between infection and minimum viral load* - **Viral load** (the amount of virus in the blood) is typically **high** shortly after infection, then decreases somewhat before rising again, and its minimum point is not directly related to the antibody window period. - The window period focuses on the host's **immune response** to the virus, specifically antibody production. *time period between infection and maximum viral load* - The **maximum viral load** usually occurs during the acute (early) phase of infection, often before the development of detectable antibodies. - This represents a peak in viral replication, not the interval until the **immune system's antibody response** becomes detectable.

Question 1030: NK cells' activity is enhanced by?

A. IL-1
B. TNF
C. IL-2 (Correct Answer)
D. TGF-β

Explanation: ***IL-2*** - **Interleukin-2** is a crucial cytokine that stimulates the proliferation and differentiation of T cells and **NK cells**, thus enhancing their cytotoxic activity. - It plays a vital role in both **adaptive and innate immune responses** by promoting NK cell maturation and increasing their ability to recognize and kill infected or cancerous cells. *IL-1* - **Interleukin-1** primarily functions as a **pro-inflammatory cytokine**, mediating systemic inflammatory responses and activating macrophages and endothelial cells. - While it modulates immune responses, its direct effect on **enhancing NK cell cytotoxicity** is not as prominent as that of IL-2. *TNF* - **Tumor Necrosis Factor (TNF)** is a cytokine involved in **systemic inflammation** and acute phase responses, also inducing apoptosis in tumor cells. - Though TNF can influence various immune cells, it is not primarily known for directly stimulating and **enhancing NK cells' activity** in the same manner as IL-2. *TGF-β* - **Transforming Growth Factor-beta (TGF-β)** is a cytokine with predominantly **immunosuppressive functions**, inhibiting the proliferation and differentiation of many immune cells, including NK cells. - It generally **downregulates immune responses** and promotes tolerance rather than enhancing NK cell activity.

Practice by Chapter

Cells and Organs of Immune System

Practice Questions

Innate Immunity

Practice Questions

Adaptive Immunity

Practice Questions

Antigens and Antibodies

Practice Questions

Major Histocompatibility Complex

Practice Questions

Complement System

Practice Questions

Cytokines and Chemokines

Practice Questions

Hypersensitivity Reactions

Practice Questions

Autoimmunity and Autoimmune Diseases

Practice Questions

Immunodeficiency Disorders

Practice Questions

Transplantation Immunology

Practice Questions

Tumor Immunology

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?