How Vaccines Work Vaccines are designed to stimulate the immune system to protect against microgranisms such as viruses. When a foreign substance invades the body, the immune system activates certain cells to destroy the invader. B cells make antibodies, molecules that attach to and neutralize viruses floating free in the bloodstream, thereby preventing the viruses from infecting other cells.
T cells can be helper cells or killer cells.
Helper T cells organize the immune response. Microorganisms such as viruses contain many molecules that are seen as foreign to the body. These different molecular shapes are called antigens, or epitopes. The B cells and T cells are activated by recognizing these antigens. Once a T cell or B cell is activated by its destiny antigen, the B or T cell clones itself, making many duplicate copies of itself.