Question

Which of the following statements about the primary and secondary adaptive immune responses to an antigen is/are correct?

• A. IgM antibodies appear first in response to the initial exposure of the antigen.
• B. Majority of the antibodies produced in response to the second exposure of the same antigen are IgM isotype.
• C. Second exposure of the same antigen stimulates production of memory cells.
• D. Primary antibody response has shorter lag phase than secondary antibody response.

Hint:

Primary response: IgM first, slower onset. Secondary response: Memory cells, IgG dominant, faster and stronger.

Answer 1

The Correct Option is A, C

Step 1: Primary response.
Upon the first exposure to an antigen:
\(\bullet\) Naive B cells are activated.
\(\bullet\) IgM is the first antibody isotype to appear.
\(\bullet\) Lag phase (time before detectable antibodies appear) is relatively long (5–7 days). Step 2: Secondary response.
Upon subsequent exposure:
\(\bullet\) Memory B and T cells respond rapidly.
\(\bullet\) Lag phase is much shorter (1–3 days), and antibody levels rise faster and higher.
\(\bullet\) IgG (and sometimes IgA or IgE depending on class switching) predominates, not IgM.
Step 3: Evaluate each statement. (A) Correct — IgM is the first antibody produced in primary response.
\ (B) Incorrect — Secondary response is dominated by IgG, not IgM.
(C) Correct — Re-exposure triggers expansion of existing memory cells.
(D) Incorrect — Secondary response has a shorter lag phase, not the primary.
Thus, correct statements are (A) and (C).