Immunology Practice Questions

Q: Which cytokine activates macrophages?

IFN-γ. ***IFN-γ (Interferon-gamma)*** - **IFN-γ is the classic macrophage-activating cytokine**, enhancing phagocytic and antimicrobial functions - Promotes expression of **MHC class I and II molecules**, increasing antigen presentation capacity - Produced mainly by **Th1 cells and NK cells** during cell-mediated immunity - Key cytokine in defense against **intracellular pathogens** (mycobacteria, viruses) *IL-8* - **IL-8 is a chemokine** (cytokine subfamily) primarily involved in **neutrophil chemotaxis** - Recruits neutrophils to sites of infection or inflammation - Does not directly activate macrophages like IFN-γ - Important in acute inflammatory responses *PAF (Platelet-Activating Factor)* - **Not a cytokine** - it is a **phospholipid mediator** - Involved in allergic and inflammatory responses - Functions include **platelet aggregation**, **vasodilation**, and **bronchoconstriction** - While it affects immune responses, it doesn't function as a macrophage-activating cytokine *Leukotriene B4* - **Not a cytokine** - it is a **lipid mediator** (eicosanoid) derived from arachidonic acid - Primarily acts as a **chemoattractant for neutrophils** - Promotes neutrophil and monocyte adhesion and migration to inflammatory sites - Does not directly activate macrophages

Q: Which of the following is considered the most professional antigen presenting cell (APC) in the immune system?

Dendritic cells. ***Dendritic cells*** - **Dendritic cells** are specialized for capture, processing, and presentation of antigens to T lymphocytes. - They are often referred to as the "**professional APCs**" due to their superior ability to initiate primary T cell responses. *B cells* - While B cells can present antigens, they are generally less efficient than **dendritic cells** and primarily serve to present antigens to **helper T cells** during secondary immune responses. - Their main role is **antibody production** after activation, not initiating primary T cell responses. *T cells* - **T cells** are effector cells of the adaptive immune system and recognize antigen presented by APCs; they do not typically function as antigen-presenting cells themselves. - Some T cells, like **gamma-delta T cells**, can present antigens, but this is not their primary role. *NK cells* - **Natural Killer (NK) cells** are part of the innate immune system and primarily target and kill infected or cancerous cells without prior sensitization. - They do not present antigens in the classical sense and are not considered professional APCs.

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 cells are known to cause rosette formation with sheep red blood cells?

T cells. ***T cells*** - **T cells** are the classic cells known to form rosettes with sheep red blood cells, a phenomenon called **E-rosette formation** - This interaction is mediated by the **CD2 receptor** on human T cells binding to **CD58 (LFA-3)** on sheep red blood cells - E-rosette formation was historically used as a diagnostic test to identify and enumerate T cells before the advent of flow cytometry - This is a characteristic feature of **mature T cells** and was widely used in immunology laboratories *NK cells* - **NK cells** do NOT typically form rosettes with sheep red blood cells - NK cells lack the specific CD2-mediated interaction required for classical E-rosette formation - NK cells are identified by other markers such as CD16 and CD56, and by their ability to kill target cells without prior sensitization *Monocytes* - **Monocytes** do not form rosettes with sheep red blood cells - Their primary functions include phagocytosis, antigen presentation, and cytokine production - They are identified by surface markers like **CD14** and their characteristic morphology (large size, kidney-shaped nucleus) *All types of T cells* - While this option is technically correct since all mature T cells express CD2 and can form E-rosettes, the more conventional answer is simply **"T cells"** - Both CD4+ helper T cells and CD8+ cytotoxic T cells possess the CD2 receptor and can participate in rosette formation - The distinction between "T cells" and "All types of T cells" is subtle, but "T cells" is the standard textbook answer

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 substances is toxic to parasites?

Peroxidase. ***Peroxidase*** - **Peroxidase** enzymes, especially those produced by **eosinophils**, generate toxic oxygen metabolites and hypohalous acids that are highly effective at damaging and killing parasites. - This enzyme plays a crucial role in the host's defense against larger parasites, such as **helminths (worms)**. *Interferon-alpha* - **Interferon-alpha** is an important cytokine primarily known for its **antiviral effects** and its role in activating natural killer (NK) cells. - While it modulates immune responses, it does not directly act as a toxic substance to parasites. *IL-2 (Interleukin-2)* - **IL-2** is a growth factor that primarily promotes the **proliferation and differentiation of T cells**, enhancing adaptive immune responses. - It does not directly kill parasites but rather supports the immune cells involved in parasite clearance. *IL-6 (Interleukin-6)* - **IL-6** is a pleiotropic cytokine involved in **inflammation, acute phase responses**, and the differentiation of B cells and T cells. - While it contributes to overall immune regulation, it lacks direct parasiticidal activity.

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 complement proteins are formed in the liver?

C2, C4. ***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.

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

IgM. ***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 1

Which cytokine activates macrophages?

Accepted Answer

IFN-γ

Answer

Leukotriene B4

Answer

IL-8

Answer

PAF

Question 2

Which of the following is considered the most professional antigen presenting cell (APC) in the immune system?

Accepted Answer

Dendritic cells

Answer

B cells

Answer

T cells

Answer

NK cells

Question 3

Which part of bacteria is most antigenic?

Accepted Answer

Lipopolysaccharide

Answer

Lipids

Answer

Nucleic acid

Answer

Protein coat

Question 4

Which cells are known to cause rosette formation with sheep red blood cells?

Accepted Answer

T cells

Answer

Monocytes

Answer

NK cells

Answer

All types of T cells

Question 5

The idiotype of an antibody is determined by -

Accepted Answer

Amino end

Answer

Fc region

Answer

Hinge region

Answer

Carboxy end

Question 6

Which of the following substances is toxic to parasites?

Accepted Answer

Peroxidase

Answer

Interferon-alpha

Answer

IL-2 (Interleukin-2)

Answer

IL-6 (Interleukin-6)

Question 7

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 8

Opsonization takes place through -

Accepted Answer

C3b

Answer

C3a (anaphylatoxin)

Answer

C5a (anaphylatoxin)

Answer

C5b (initiates MAC formation)

Question 9

Which complement proteins are formed in the liver?

Accepted Answer

C2, C4

Answer

C3, C6

Answer

C5, C8

Answer

C1

Question 10

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

Accepted Answer

IgM

Answer

IgA

Answer

IgG

Answer

IgD

Immunology — MCQs

Immunology — MCQs

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