In the lymph nodes, a mature B cell, activated by helper T cells and related antigen, “proliferates and differentiates to form plasma cells that secrete identical antibodies with a single specificity at a rate of about 10,000 molecules per cell per second.”​—Immunology.

Many of them become plasma cells. Again, there must be the right receptors on the helper T cells to join up with the B cells and cause them to produce plasma cells. It is those plasma cells that start churning out thousands of antibodies a second.

Since each plasma cell makes only one kind of antibody, with a receptor specific for only one disease antigen, soon billions are on the front lines homing in on the antigens of one specific disease.