Question

Which class of antibody appears first in serum after exposure to an antigen?

• A. Ig M
• B. Ig G
• C. Ig A
• D. Ig D

Hint:

Humoral Kinetics:
First antibody produced in primary response = IgM.
Predominant antibody in secondary (anamnestic) response = IgG.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Primary humoral immune kinetics feature early synthesis of pentameric IgM followed by isotype class switching to high-affinity IgG.

Step 2: Detailed Explanation:
In the humoral immune response:
1. Primary Immune Response: Upon initial exposure to a novel immunogen, mature naïve B cells are activated and differentiate into short-lived plasma cells that secrete Immunoglobulin M (IgM) as a high-avidity pentamer. IgM appears first in serum after a lag phase of 5 to 7 days.
2. Secondary Late Response: Helper T-cell derived cytokines induce immunoglobulin class switching to IgG (which dominates the secondary memory response).

Step 3: Final Answer:
Thus, the antibody class appearing first in serum is IgM, matching option (A).