Vaccines are the second-largest category of over 200 drugs now being produced by American pharmaceutical companies using biotechnology. Other products include hormones, interferons, blood clotting factors, antisense molecules, and enzymes. Most of these drugs are still undergoing clinical testing, and are designed to combat cancer, AIDS, asthma, diabetes, heart disease, Lyme disease, multiple sclerosis, rheumatoid arthritis, and viral infections. Outnumbering every other type of biotech medical product, however, are monoclonal antibodies—a versatile group of molecules with seemingly endless applications.

The ideal drug, said German medical pioneer Paul Ehrlich (1854-1915), would target and eliminate infections while having no ill effects in patients. It would be like a magic bullet, unfailingly hitting the bull's-eye. While no such drugs have yet been made (even the best have occasional harmful side effects), nature makes something very like them in antibodies.

Antibodies are part of the body's immune system. Manufactured by B-cells in the spleen, blood, and lymph glands, antibodies are proteins that latch onto invading microbes or other foreign materials, tagging them for destruction by other body cells. Anything that stimulates antibody production is called an antigen.

Each B-cell produces an antibody molecule shaped to fit precisely to the surface of a particular antigen, like a hand in a glove. Since antigens come in all shapes and sizes, the body is able to produce literally millions of different types of antibodies, each specific to an antigen and