B cells have a slightly larger version of the antibody protein on its cell surface. When a B cell binds to an antigen that fits in its cell surface antibody precisely, it waits for a T cell to give a second signal that triggers maximal activation. It takes about 12 hours for a B cell to double in size and divide into two cells, and when an infection is detected, this process usually goes on for about a week or so. Each plasma cell can produce more than 2000 antibody molecules per second. Some of the B cells become memory cells instead of plasma cells.
Plasma cells die after about two days, but memory B cells live for decades.
At this point, the plasma cells die out, but lots of memory cells remain. This vigorous secondary response constitutes immunologic memory and prevents the invading pathogen from getting beyond the very initial stages of infection. Most abundant class of antibody in serum, can cross the placenta to protect the fetus, can activate complement and phagocytosis. IgM is secreted as a pentamer. Five IgMs are linked by disulfide bonds and a small protein called the J chain.