Immunology Practice Questions

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 not encoded by the MHC Class III region?

Complement C3. The **Major Histocompatibility Complex (MHC)** is a large gene cluster on chromosome 6. While Class I and II genes encode cell surface molecules involved in antigen presentation, the **Class III region** encodes a diverse group of secreted proteins involved in the immune and inflammatory response. ### Why Complement C3 is the Correct Answer **Complement C3** is not encoded within the MHC locus. It is encoded by a gene located on **Chromosome 19**. While the MHC Class III region does encode several components of the complement system (specifically C2, C4A, C4B, and Factor B), C3 is a notable exception and a common "trap" in PG entrance exams. ### Analysis of Incorrect Options * **Properdin:** While historically some texts debated its location, modern genomic mapping confirms that genes for several alternative pathway components and inflammatory regulators are associated with the MHC III region. However, in the context of this classic MCQ, **TNF** and **HSPs** are the definitive MHC III products. * **TNF-alpha:** The genes for Tumor Necrosis Factor (TNF-α) and Lymphotoxin (TNF-β) are located within the MHC Class III region, between the Class I and Class II clusters. * **Heat Shock Proteins (HSPs):** Specifically, the **HSP70** family genes are located within the MHC Class III region. They act as molecular chaperones and are involved in protein folding and cytoprotection. ### High-Yield Clinical Pearls for NEET-PG * **MHC Location:** Short arm of **Chromosome 6 (6p)**. * **MHC Class III Products:** Remember the mnemonic **"C-T-H"**: 1. **C**omplement proteins (C2, C4, Factor B). 2. **T**umor Necrosis Factors (TNF-α, TNF-β). 3. **H**eat Shock Proteins (HSP70). * **Key Distinction:** MHC Class I and II molecules are membrane-bound, whereas MHC Class III molecules are mostly **secreted proteins** and do not participate in antigen presentation.

Q: What is the predominant immunoglobulin found in saliva?

IgA. **Explanation:** **Correct Option: A (IgA)** Immunoglobulin A (IgA) is the predominant antibody found in seromucous secretions, including **saliva**, tears, colostrum, and gastrointestinal/respiratory secretions. In these fluids, it exists primarily as **Secretory IgA (sIgA)**, which is a dimer. It contains a **J-chain** (joining chain) and a **secretory component** that protects the antibody from degradation by proteolytic enzymes in the harsh environments of the gut and mouth. Its primary function is "immune exclusion," preventing the attachment of pathogens to mucosal surfaces. **Why other options are incorrect:** * **IgD:** Primarily found on the surface of B-lymphocytes where it acts as an antigen receptor. It is present in only trace amounts in serum and secretions. * **IgE:** Involved in type I hypersensitivity (allergic) reactions and defense against helminthic parasites. It is found in very low concentrations in the blood and is typically bound to mast cells and basophils. * **IgG:** The most abundant immunoglobulin in the **serum** (approx. 75-80%) and the only one that crosses the placenta. While it can be found in saliva via gingival crevicular fluid, it is not the predominant type. **High-Yield NEET-PG Pearls:** * **Structure:** IgA is a monomer in serum but a **dimer** in secretions. * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent sinopulmonary and GI infections. * **Breast Milk:** IgA is the most abundant antibody in colostrum, providing passive immunity to the neonate's gut. * **Half-life:** IgG has the longest half-life (approx. 23 days), making it the mainstay of secondary immune responses.

Q: Neisseria infections are associated with which of the following?

Deficiency of late complements. **Explanation:** The correct answer is **B. Deficiency of late complements**. **Why it is correct:** The late complement components (**C5, C6, C7, C8, and C9**) assemble to form the **Membrane Attack Complex (MAC)**. The MAC is essential for the lysis of Gram-negative bacteria with thin peptidoglycan layers. *Neisseria* species (*N. meningitidis* and *N. gonorrhoeae*) are uniquely susceptible to MAC-mediated killing. Patients with deficiencies in these terminal components cannot form the MAC effectively, leading to a significantly increased risk (up to 10,000-fold) of recurrent disseminated Neisserial infections. **Why other options are incorrect:** * **Option A:** Deficiency of **early complements** (C1, C2, C4) is primarily associated with **immune complex diseases** like Systemic Lupus Erythematosus (SLE) and pyogenic infections (e.g., *S. pneumoniae*), as they are required for opsonization and clearance of immune complexes. * **Option C:** There is a well-documented clinical association between complement pathways and specific bacterial vulnerabilities. * **Option D:** While C3 deficiency is severe and increases risk for many infections, the specific, classic association for *Neisseria* is specifically with the terminal/late components. **High-Yield Clinical Pearls for NEET-PG:** * **C3 Deficiency:** The most severe complement deficiency; leads to recurrent infections with pyogenic (encapsulated) bacteria and Type III hypersensitivity reactions. * **C1 Esterase Inhibitor Deficiency:** Causes **Hereditary Angioedema** (characterized by low C4 levels). * **CH50 Assay:** Used to screen for classical pathway deficiencies; it will be low/zero in terminal component deficiencies. * **Properdin/Factor D Deficiency:** Associated with *Neisseria* due to impairment of the alternative pathway.

Question 1

A 28-year-old male known to be HIV-positive with a CD4 count of 45 cells/µL presents with 2 weeks of progressive headache, photophobia, and neck stiffness. CSF opening pressure is markedly elevated. CSF microscopy is shown (Image 2). Which of the following statements most accurately describes the virulence mechanism that directly accounts for the markedly elevated CSF opening pressure seen in this patient?

Accepted Answer

A. The polysaccharide capsule inhibits phagocytosis and impairs CSF absorption at the arachnoid villi by accumulating in the subarachnoid space

Answer

B. Production of phospholipase B disrupts the blood-brain barrier, causing vasogenic cerebral oedema

Answer

C. Melanin synthesis via laccase enzyme causes direct cytotoxic damage to choroid plexus epithelium

Answer

D. Urease-mediated ammonia production raises local CSF pH, triggering reactive ependymal inflammation and obstructive hydrocephalus

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 not encoded by the MHC Class III region?

Accepted Answer

Complement C3

Answer

Properdin

Answer

TNF-alpha

Answer

Heat shock proteins

Question 4

What is the predominant immunoglobulin found in saliva?

Accepted Answer

IgA

Answer

IgD

Answer

IgE

Answer

IgG

Question 5

Neisseria infections are associated with which of the following?

Accepted Answer

Deficiency of late complements

Answer

Deficiency of early complements

Answer

There is no such association

Answer

Any complement deficiency can be associated

Immunology — MCQs

Immunology — MCQs

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