B cells, a type of lymphocyte (white blood cell), have an important role in the body's immune system. When a B cell encounters an invading bacterium it starts to divide, forming two different types of cell. One type is a clone of itself that begins to produce antibodies to fight the infection; the other is a memory cell that will persist in the bloodstream, ready to produce antibodies should re-infection occur.

T cells, a type of lymphocyte (white blood cell), have an important role in the body's immune system. When a T cell encounters an invading virus it begins to divide, forming four different types of T cell, each with a different function. Killer T cells destroy cells that have become infected with the virus by lysis. Helper T cells activate more killer T cells and also stimulate B cells to begin antibody production (unlike B cells, T cells do not produce antibodies to destroy invading pathogens). Suppressor T cells protect healthy cells from viral attack and memory T cells persist in the bloodstream to guard against re-infection.