Which best characterizes the secondary immune response?

• A. Equal amounts of IgM and IgG are produced.
• B. There is an increase in IgM only.
• C. There is a large increase in IgG but not IgM.
• D. The lag phase is the same as in the primary response.

Answer: (C)

In a secondary immune response, memory B cells from the first exposure react quickly and mainly produce IgG. This rapid response comes from class switching and affinity maturation that occurred during the primary response, so the body can generate high-affinity IgG antibodies fast. IgM may be produced, but it is not the dominant antibody in this phase, and the overall antibody titer of IgG rises to much higher levels than in the primary response. The lag phase is shorter than in the first encounter.

So the description that fits best is a large increase in IgG but not IgM. The other patterns—equal IgM and IgG, IgM-only increase, or a lag phase unchanged from the primary—don’t align with how memory B cells drive a quick, IgG-dominant secondary response.