The graph shows the level of antibody in serum following vaccination and a challenge with the same antigen 28 days later.
Study the graphs below and answer the question:

What does peak A represents?

Explanation

Level of IgM only. In a primary immune response, naive B cells are stimulated by antigen, become activated, and differentiate into antibody-secreting cells that produce antibodies specific for the eliciting antigen. A large amount of IgM and a small amount of IgG are produced during the primary immune response.