Question

A secondary immune response

• A. Can be induced by a single dose of antigen
• B. Occurs after several months
• C. Has a very short lag period
• D. Occurs only against microbial toxins

Hint:

The primary immune response is dominated by IgM with a lag phase of 5--10 days, while the secondary response is dominated by IgG with a lag phase of only 1--3 days.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The immune system exhibits memory upon repeated exposure to the same antigen.
The primary immune response occurs on the first exposure, characterized by a prolonged lag phase as naive lymphocytes undergo clonal selection and differentiation.
The secondary (or anamnestic) response occurs during subsequent exposures and is faster, stronger, and more prolonged due to the presence of memory B and T cells.
Detailed Explanation:
During a primary immune response, it takes 5 to 10 days (the lag phase) for detectable antibodies to appear in the serum.
This delay occurs because naive B and T cells must recognize the antigen, undergo clonal expansion, and differentiate into plasma cells and effector T cells.
In contrast, a secondary immune response is mediated by pre-existing antigen-specific memory cells.
These memory cells are long-lived, exist in higher precursor frequencies, and are more easily activated than naive cells.
Consequently, the lag period is significantly shortened to 1 to 3 days.
The antibody titer rises rapidly, reaches a much higher peak, and consists primarily of high-affinity IgG (due to affinity maturation and class switching) rather than IgM, which dominates the primary response.
This response is not restricted to microbial toxins and requires previous exposure (cannot be induced by a single dose of antigen in a naive individual).

Step 2: Final Answer:
The defining characteristic of a secondary immune response is its very short lag period.