Immunology MCQs (1-10): Answers with Explanations

Immunology Practice Questions (1–10)

1. Specific immunity can be acquired either naturally or artificially and involves:

a. All of the below

b. Antibodies

c. Antigens

d. The classical complement pathway

2. Which DOES NOT provide long-term immunity?

a. Artificially acquired active immunity

b. Artificially acquired passive immunity

c. Naturally acquired active immunity

d. None of the above

3. An epitope is:

a. A B-cell

b. A hapten

c. An antibody

d. The antigen determinant site

4. An immunoglobulin is a:

a. Carbohydrate

b. Fatty acid

c. Glycoprotein

d. Protein

5. Isotypes refer to variations in the:

a. Heavy chain constant region

b. Heavy chain variable region

c. Light chain constant region

d. Light chain variable region

6. The Fc region mediates all of the below EXCEPT binding to:

a. Antigen

b. Lysosomes

c. Some phagocytic cells

d. Various cells of the immune system

7. Which of the following is the major immunoglobulin in human serum, accounting for 80% of the immunoglobulin pool?

a. IgA

b. IgD

c. IgG

d. IgM

8. Which immunoglobulin is the least prevalent?

a. IgA

b. IgD

c. IgE

d. IgM

9. Which of the following is useful to STIMULATE antibody production?

a. An adjuvant

b. A hapten

c. Antiserum

d. Purified antigen

e. Crude antigen

10. Compared to the secondary antibody response, the primary response:

a. Attains a higher IgG titer

b. Has a longer lag phase

c. Persists for a longer plateau period

d. Produces antibodies with a higher affinity for the antigen

Answer Key & Explanations

**1. Correct Answer: a (All of the below)**

• Explanation: Specific (adaptive) immunity relies fundamentally on the interaction between foreign antigens and host-produced antibodies. Furthermore, when these specific antibodies bind to their target antigens, they trigger the classical complement pathway to clear the pathogen, making all the listed options integral components of the adaptive response.

**2. Correct Answer: b (Artificially acquired passive immunity)**

• Explanation: Passive immunity involves the direct transfer of pre-formed antibodies (such as administering intravenous immunoglobulin or anti-venom) into a recipient. Because the host's own immune system is never activated to undergo clonal expansion or produce memory cells, protection disappears as soon as the infused proteins are naturally metabolized.

**3. Correct Answer: d (The antigen determinant site)**

• Explanation: An epitope is the exact structural and chemical site on the surface of a macromolecular antigen that is recognized and bound by a specific B-cell receptor, T-cell receptor, or antibody molecule. It is the minimal unit that defines antigenic specificity.

**4. Correct Answer: c (Glycoprotein)**

• Explanation: Biochemically, immunoglobulins are classified as glycoproteins. They consist of a basic polypeptide structure containing two heavy and two light chains held together by disulfide bonds, interspersed with covalently attached carbohydrate groups localized primarily along the constant regions.

**5. Correct Answer: a (Heavy chain constant region)**

• Explanation: Immunoglobulin isotypes represent the structural variations within the constant regions of the heavy chains that are common to all healthy individuals of a species. These variations determine the broad class of the antibody (IgM, IgG, IgA, IgE, or IgD) and dictate its structural arrangement and biological effector pathways.

**6. Correct Answer: a (Antigen)**

• Explanation: The Fc (Fragment crystallizable) region of an antibody does not bind to antigens. Antigen binding is exclusively handled by the two Fab (Fragment antigen-binding) regions, which contain the hypervariable loops. Instead, the Fc tail anchors into Fc receptors found on phagocytes, mast cells, and lysosomes, and initiates complement activation.

**7. Correct Answer: c (IgG)**

• Explanation: IgG is the dominant antibody class circulating in human blood serum and extravascular tissues, structurally accounting for approximately 80% of the total immunoglobulin pool. Its abundance allows it to serve as the baseline shield against circulating bacteria, toxins, and viruses.

**8. Correct Answer: c (IgE)**

• Explanation: IgE is the least prevalent antibody class in normal human serum, often present in trace concentrations. This is because IgE molecules are heavily sequestered out of circulation, being tightly bound to high-affinity Fc-epsilon receptors located on the surfaces of tissue mast cells and circulating basophils.

**9. Correct Answer: a (An adjuvant)**

• Explanation: An adjuvant is a substance formulated into a vaccine mixture to non-specifically enhance and stimulate a more robust antibody production. It works by extending the biological half-life of the antigen (depot effect) or by directly stimulating local innate immune cells via pattern recognition pathways. A hapten cannot stimulate an immune response on its own, and antiserum provides passive antibodies rather than stimulating production.

**10. Correct Answer: b (Has a longer lag phase)**

• Explanation: When the immune system encounters a novel antigen for the very first time, it lacks an established army of memory cells. Consequently, the primary immune response requires a longer lag phase (often 3 to 5 days) for naive lymphocytes to recognize the antigen, undergo clonal expansion, and differentiate into effector plasma cells. Conversely, secondary responses feature a rapid lag phase, higher affinity, and much higher titers.