As long ago as the 5th century B.C., Greek physicians noted that people who had recovered from the plague would never get it again-they had acquired immunity. This is because, whenever T cells and B cells are activated, some of the cells become "memory" cells. Then, the next time that an individual encounters that same antigen, the immune system is primed to destroy it quickly.

The degree and duration of immunity depend on the kind of antigen, its amount, and how it enters the body. An immune response is also dictated by heredity; some individuals respond strongly to a given antigen, others weakly, and some not at all.

Infants are born with relatively weak immune responses. They have, however, a natural "passive" immunity; they are protected during the first months of life by means of antibodies they receive from their mothers. The antibody IgG, which travels across the placenta, makes them immune to the same microbes to which their mothers are immune. Children who are nursed also receive IgA from breast milk; it protects the digestive tract.

Passive immunity can also be conveyed by antibody-containing serum obtained from individuals who are immune to a specific infectious agent. Immune serum globulin or "gamma globulin" is sometimes given to protect travelers to countries where hepatitis is widespread. Passive immunity typically lasts only a few weeks.

"Active" immunity-mounting an immune response-can be triggered by both infection and vaccination. Vaccines contain microorganisms that have been altered so they will produce an immune response but will not be able to induce full-blown disease. Some vaccines are made from microbes that have been killed. Others use microbes that have been changed slightly so they can no longer produce infection. They may, for instance, be unable to multiply. Some vaccines are made from a live virus that has been weakened, or attenuated, by growing it for many cycles in animals or cell cultures.

Recent research, benefiting from the biotechnology revolution, has focused on developing vaccines that use only part of the infectious agent. Such subunit vaccines , which are now available for meningitis, pneumonia, and hepatitis B, produce the desired immunity without stirring up separate and potentially harmful immune reactions to the many antigens carried, for instance, on a single bacterium.