monoclonal antibody, an antibody that is mass produced in the laboratory from a single clone and that recognizes only one antigen. Monoclonal antibodies are typically made by fusing a normally short-lived, antibody-producing B cell (see immunity) to a fast-growing cell, such as a cancer cell (sometimes referred to as an "immortal" cell). The resulting hybrid cell, or hybridoma, multiplies rapidly, creating a clone that produces large quantities of the antibody.

Monoclonal antibodies engendered much excitement in the medical world and in the financial world in the 1980s, especially as potential cures for cancer. They have been used in laboratory research and in medical tests since the mid-1970s, but their effectiveness in disease treatment has been limited. By the mid-1990s, however, some of the technical problems had been overcome. Experimental cancer therapies have used drugs, radioactive materials, or immune killer cells attached to monoclonal antibodies that, when injected into patients, home in on antigens that grow only on the surface of cancer cells.

antibody, protein produced by the immune system (see immunity) in response to the presence in the body of antigens: foreign proteins or polysaccharides such as bacteria, bacterial toxins, viruses, or other cells or proteins. Such antigens are capable of inflicting damage by chemically combining with natural substances in the body and disrupting the body's processes. The body contains hundreds of thousands of different white blood cells called B lymphocytes, each capable of producing one type of antibody and each bearing sites on its membrane that will bind with a specific antigen. When such a binding occurs, it triggers the B lymphocyte to reproduce itself, forming a clone that manufactures vast amounts of its antibody.

The antibody molecule is composed of four polypeptide chains (see peptide)—two identical light chains and two identical heavy chains—joined by disulfide bridges. The light chains have a variable portion that is different in each type of antibody and is the active portion of the molecule that binds with the specific antigen. Antibodies combine with some antigens, such as bacterial toxins, and neutralize their effect; they remove other substances from circulation in body fluids; they bind certain antigens together, a process known as agglutination; and they activate complement, blood serum proteins that cause the destruction of invading cells.

See also monoclonal antibody.

Molecule in the immune system that circulates in blood and lymph in response to invasion by an antigen. Antibodies are globulins formed in lymphoid tissues by B cells, whose receptors are specialized to bind to a specific antigen. These receptors are copied as antibodies that attack the target antigens by binding to them, either neutralizing them or triggering a complement reaction. Antibodies have widely varying binding sites, providing protection from a wide range of infectious agents and toxic substances. Antibodies derived from the blood serum of infected people or animals are often given in an antiserum for quick immunization against fast-acting toxins or microbes. In 1975 César Milstein and colleagues developed a process for producing specific antibodies in virtually limitless amounts; these monoclonal antibodies can deliver radiation or drugs directly to specific antigens. Seealso antitoxin; reticuloendothelial system.

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