What is the difference between primary and secondary immune responses in terms of antibody isotype and speed?

• A. Primary response is slower with initial IgM production; secondary response is faster with higher-affinity IgG due to memory cells
• B. Primary response is faster with IgG; secondary response is slower with IgM
• C. Primary uses IgA; secondary uses IgE
• D. Primary yields only IgM; secondary yields only IgG, both slow

Answer: (A)

The key idea is how the immune system responds differently to a first encounter versus a later encounter with the same antigen. On the first exposure, naive B cells are activated and start producing antibodies, beginning with IgM. This response develops slowly, because it takes time to activate and expand the right B cells, and the antibodies produced early on are of lower affinity.

When the same antigen is encountered again, memory B cells generated from the first response kick in. These memory cells rapidly differentiate into plasma cells and produce antibodies much faster and in greater quantities. The antibodies are predominantly IgG and have higher affinity due to class switching and affinity maturation, so they bind the antigen more effectively.

So the best description is: the primary response is slower and starts with IgM; the secondary response is faster and produces higher-affinity IgG because memory B cells are primed to respond quickly.