Immunology Practice Questions

Q: Which is the most immunogenic antigen of Salmonella Typhi?

H antigen. **Explanation:** The immunogenicity of an antigen is determined by its chemical complexity and size. In *Salmonella Typhi*, the **H (Flagellar) antigen** is the most immunogenic because it is composed of proteins (flagellin). Proteins are more potent triggers of the immune system compared to polysaccharides, leading to a robust antibody response. This is why H-agglutinins appear earlier and reach higher titers than O-agglutinins during a *Salmonella* infection. **Analysis of Options:** * **A & D. O Antigen (Somatic Antigen):** These are the same entity. The O antigen is a lipopolysaccharide (LPS) located on the outer membrane. While it is important for serogrouping, polysaccharides are generally less immunogenic than proteins. O-antibodies appear later and disappear sooner than H-antibodies. * **C. Vi Antigen:** This is a surface polysaccharide capsular antigen (Virulence antigen). It is poorly immunogenic and primarily functions by masking the O antigen from antibodies. Its main clinical utility is in identifying chronic carriers and for use in certain vaccines (e.g., Typhim VI). **High-Yield Clinical Pearls for NEET-PG:** * **Widal Test:** Measures antibodies against O and H antigens. A titer of **>1:160 for O** and **>1:160 for H** is usually considered significant in endemic areas. * **Sequence of Appearance:** In Enteric fever, O antibodies appear first (around the end of the 1st week), but H antibodies reach higher peaks and persist longer. * **Carrier State:** Persistent high titers of **Vi antibodies** (1:10 or more) suggest a chronic carrier state, as the bacteria continue to harbor the capsule in the gallbladder or urinary tract.

Q: Which of the following is true regarding lattice formation?

Associated with both precipitation and agglutination. **Explanation:** The **Lattice Hypothesis**, proposed by Marrack (1934), is the fundamental principle governing all visible antigen-antibody (Ag-Ab) reactions. It states that for a visible reaction to occur, multivalent antigens must be cross-linked by bivalent antibodies to form a large, insoluble three-dimensional network or "lattice." **Why Option C is correct:** Lattice formation is the prerequisite for both **precipitation** and **agglutination**. * In **precipitation**, the antigen is **soluble**. When it reacts with its specific antibody at the "Zone of Equivalence," they form a lattice that becomes too large to remain in solution and settles as a visible precipitate. * In **agglutination**, the antigen is **particulate** (e.g., bacteria, RBCs). The antibodies act as bridges between these particles, forming a lattice that results in visible clumping. **Why other options are incorrect:** * **Options A & B:** These are incorrect because they suggest the lattice phenomenon is exclusive to one type of reaction. While the physical state of the antigen differs (soluble vs. particulate), the underlying mechanism of cross-linking to form a lattice remains identical. * **Option D:** This is incorrect as no visible Ag-Ab reaction can occur without the formation of a lattice. **NEET-PG High-Yield Pearls:** 1. **Zone of Equivalence:** This is the specific ratio of Ag to Ab where lattice formation is maximal. 2. **Prozone Phenomenon:** False negative result due to **antibody excess**. No lattice forms because every antigenic site is saturated by a single antibody, preventing cross-linking. 3. **Postzone Phenomenon:** False negative result due to **antigen excess**. 4. **Valency:** For a lattice to form, the antigen must be multivalent and the antibody must be at least bivalent (IgG) or multivalent (IgM).

Q: Which of the following is a superantigen?

TSST. **Explanation:** **Correct Answer: C. TSST (Toxic Shock Syndrome Toxin-1)** **Mechanism of Superantigens:** Superantigens are unique proteins that bypass the conventional antigen-processing pathway. Unlike regular antigens that bind to the antigen-binding groove of MHC II molecules, superantigens bind **externally** to the **Vβ region of T-cell receptors (TCR)** and the **MHC II** on antigen-presenting cells. This results in a non-specific, massive activation of T-cells (up to 20% of the total T-cell pool), leading to a "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ). This systemic inflammatory response causes the clinical features of Toxic Shock Syndrome: fever, hypotension, and multi-organ failure. **Analysis of Incorrect Options:** * **A. Cholera toxin:** An A-B type enterotoxin produced by *Vibrio cholerae*. It acts by ADP-ribosylation of Gs proteins, increasing cAMP levels, leading to secretory diarrhea. * **B. Diphtheria toxin:** An A-B toxin produced by *Corynebacterium diphtheriae*. It inhibits protein synthesis by ADP-ribosylation of Elongation Factor-2 (EF-2). * **D. Vero-cytoxin (Shiga-like toxin):** Produced by EHEC (e.g., O157:H7). It inhibits the 60S ribosomal subunit, leading to cell death and Hemolytic Uremic Syndrome (HUS). **High-Yield Facts for NEET-PG:** * **Common Superantigens:** * *Staph. aureus:* TSST-1, Exfoliative toxin (Scalded Skin Syndrome), and Enterotoxins (Food poisoning). * *Strep. pyogenes:* Erythrogenic toxin (SpeA and SpeC) causing Scarlet fever. * **Key Feature:** Superantigens do **not** require processing by macrophages; they bind directly to the outside of the MHC II-TCR complex. * **Consequence:** Massive release of **TNF-α** is the primary driver of shock in these patients.

Q: CD8 antigen is present on which type of immune cell?

T suppressor cells. **Explanation:** The **CD8 antigen** is a transmembrane glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). It specifically recognizes and binds to **MHC Class I** molecules. **Why T suppressor cells are correct:** T lymphocytes are broadly divided into two subsets based on their surface markers. CD8+ T cells primarily differentiate into two functional types: **Cytotoxic T cells (Tc)** and **Suppressor T cells (Ts)**. While Cytotoxic T cells directly kill virally infected or tumor cells, Suppressor T cells play a crucial role in downregulating the immune response and maintaining self-tolerance. Therefore, CD8 is the hallmark marker for T suppressor cells. **Analysis of Incorrect Options:** * **A. T helper cells:** These are characterized by the **CD4** antigen, which interacts with MHC Class II molecules. * **B. B cells:** These cells do not express CD8. Their characteristic markers include **CD19, CD20, and CD21**. * **C. Macrophages:** These are professional antigen-presenting cells (APCs) that express **CD14** and MHC Class II. While they may express low levels of CD4, they do not express CD8. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (Rule of 8):** CD4 × MHC II = 8; CD8 × MHC I = 8. * **CD4:CD8 Ratio:** In a healthy individual, the normal ratio is approximately **2:1**. This ratio is characteristically **reversed (<1)** in HIV/AIDS due to the depletion of CD4+ T cells. * **MHC Restriction:** CD8+ cells are "MHC Class I restricted," meaning they only recognize endogenous antigens presented on MHC I (found on all nucleated cells).

Q: Immunoglobulins are secreted by which of the following cells?

Plasma cells. **Explanation:** **Why Plasma Cells are the Correct Answer:** Immunoglobulins (antibodies) are produced and secreted exclusively by **Plasma cells**. These cells are the final functional stage of **B-lymphocyte** differentiation. When a B-cell encounters a specific antigen and receives necessary T-cell help, it undergoes clonal expansion and differentiates into plasma cells. These cells are specialized "protein factories" characterized by an extensive rough endoplasmic reticulum (RER) and a prominent Golgi apparatus, allowing them to synthesize and secrete thousands of antibody molecules per second. **Analysis of Incorrect Options:** * **A. Macrophages:** These are professional phagocytes derived from monocytes. Their primary role is engulfing pathogens and acting as **Antigen-Presenting Cells (APCs)**, not antibody production. * **C. T-cells:** These are responsible for **Cell-Mediated Immunity**. While they regulate the immune response (Helper T-cells) or kill infected cells directly (Cytotoxic T-cells), they do not secrete antibodies. * **D. Neutrophils:** These are the first responders to acute inflammation. They eliminate pathogens via phagocytosis, degranulation, and the formation of Neutrophil Extracellular Traps (NETs). **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** On histology, plasma cells exhibit a characteristic **"Cart-wheel" or "Clock-face" nucleus** and a **perinuclear halo** (representing the Golgi apparatus). * **Malignancy:** A clonal proliferation of plasma cells leads to **Multiple Myeloma**, characterized by the production of monoclonal (M) proteins. * **Russell Bodies:** These are eosinophilic inclusions found in plasma cells undergoing excessive immunoglobulin synthesis. * **B-cell Marker:** CD19, CD20, and CD21 are markers for B-cells, but mature plasma cells often lose CD20 and express **CD138**.

Question 1

Which is the most immunogenic antigen of Salmonella Typhi?

Accepted Answer

H antigen

Answer

O antigen

Answer

Vi antigen

Answer

Somatic antigen

Question 2

Which of the following is true regarding lattice formation?

Accepted Answer

Associated with both precipitation and agglutination

Answer

Associated with precipitation and not agglutination

Answer

Associated with agglutination and not precipitation

Answer

Associated with neither precipitation nor agglutination

Question 3

Which of the following is a superantigen?

Accepted Answer

TSST

Answer

Cholera toxin

Answer

Diphtheria toxin

Answer

Vero-cytoxin

Question 4

CD8 antigen is present on which type of immune cell?

Accepted Answer

T suppressor cells

Answer

T helper cells

Answer

B cells

Answer

Macrophages

Question 5

Immunoglobulins are secreted by which of the following cells?

Accepted Answer

Plasma cells

Answer

Macrophages

Answer

T-cells

Answer

Neutrophils

Immunology — MCQs

Immunology — MCQs

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