Immunology Practice Questions

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

Interferons inhibit viral replication in host cells.. **Interferons inhibit viral replication in host cells.** - Interferons are a crucial part of the **innate immune response** against viral infections. - They work by inducing an **antiviral state** in neighboring uninfected cells, preventing viral replication. - Interferons are **non-specific** antiviral cytokines with broad-spectrum activity. *Interferons are specific for particular viruses.* - This is incorrect. Interferons are **virus-non-specific** in their antiviral action. - While their production is triggered by viral infection, they can inhibit replication of various different viruses. - This broad-spectrum activity is a key feature distinguishing them from antibodies (which are specific). *Interferons have no effect on viral infections.* - This is false. Interferons are potent **antiviral cytokines** essential for host defense against viruses. - They activate immune cells (NK cells, macrophages) and induce expression of genes that inhibit viral processes. *Interferons are synthetic antiviral agents.* - This is incorrect. Interferons are **naturally produced proteins** by the body's cells (leukocytes, fibroblasts, dendritic cells) in response to viral infections. - While recombinant interferons are used therapeutically, interferons are biological molecules, not synthetic drugs.

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

C3 (complement component 3). ***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.

Q: When is the prozone phenomenon seen?

Antibody excess to antigen. ***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.

Q: The idiotype of an antibody is determined by -

Amino end. ***Amino end*** - The **idiotypic class** of an antibody is determined by the unique amino acid sequences within the **variable regions** of both the heavy and light chains, located at the **amino-terminal end** of the antibody. - These variable regions form the **antigen-binding sites**, giving each antibody its specific binding capabilities and thus its idiotype. *Fc region* - The **Fc (fragment crystallizable) region** is the tail region of an antibody that interacts with cell surface receptors and other immune system molecules. - It determines the **isotype (class)** of the antibody (e.g., IgG, IgM) and mediates effector functions but does not define the idiotype. *Hinge region* - The **hinge region** is a flexible segment in the middle of the heavy chains of IgG, IgA, and IgD antibodies. - It allows flexibility between the antigen-binding (Fab) arms, facilitating bivalent binding, but does not determine the idiotype. *Carboxy end* - The **carboxy end** (C-terminus) of the heavy and light chains typically corresponds to the constant regions of the antibody. - While it contributes to the antibody's overall structure and effector functions (especially the Fc region), it does not contain the unique sequences that define the **idiotype**.

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

They primarily activate B-cells.. ***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

Q: Opsonization takes place through -

C3b. ***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.

Q: Which part of bacteria is most antigenic?

Lipopolysaccharide. ***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

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

Mature dendritic cells. ***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.

Q: HIV window period indicates:

time period between infection and detection of antibodies. ***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 1

Which of the following statements about interferons is true?

Accepted Answer

Interferons inhibit viral replication in host cells.

Answer

Interferons are specific for particular viruses.

Answer

Interferons have no effect on viral infections.

Answer

Interferons are synthetic antiviral agents.

Question 2

Which component is considered the central part of the complement pathway?

Accepted Answer

C3 (complement component 3)

Answer

C1 (complement component 1)

Answer

C2 (complement component 2)

Answer

C5 (complement component 5)

Question 3

When is the prozone phenomenon seen?

Accepted Answer

Antibody excess to antigen

Answer

Same concentration of antibody and antigen

Answer

Hyperimmune reaction

Answer

In antigen excess to antibody

Question 4

The idiotype of an antibody is determined by -

Accepted Answer

Amino end

Answer

Fc region

Answer

Hinge region

Answer

Carboxy end

Question 5

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

Accepted Answer

They primarily activate B-cells.

Answer

They primarily activate T-cells.

Answer

They primarily activate macrophages.

Answer

They primarily activate CD8+ T cells.

Question 6

Opsonization takes place through -

Accepted Answer

C3b

Answer

C3a (anaphylatoxin)

Answer

C5a (anaphylatoxin)

Answer

C5b (initiates MAC formation)

Question 7

Which part of bacteria is most antigenic?

Accepted Answer

Lipopolysaccharide

Answer

Lipids

Answer

Nucleic acid

Answer

Protein coat

Question 8

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

Accepted Answer

Mature dendritic cells

Answer

Macrophages

Answer

B-cell

Answer

Follicular dendritic cells

Question 9

HIV window period indicates:

Accepted Answer

time period between infection and detection of antibodies

Answer

time period between infection and onset of first symptoms

Answer

time period between infection and minimum viral load

Answer

time period between infection and maximum viral load

Question 10

All of the following are pattern recognition receptors for extracellular or ingested microbes, except:

Accepted Answer

Killer-cell immunoglobulin receptors (KIRs) - Recognize MHC class I molecules on host cells.

Answer

Toll-like receptors (TLRs) - Detect pathogen-associated molecular patterns (PAMPs).

Answer

NOD-like receptors (NLRs) - Intracellular sensors for microbial components.

Answer

C-type lectin receptors (CLRs) - Recognize carbohydrate structures on microbes.

Immunology — MCQs

Immunology — MCQs

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