Molecule in the immune system that circulates in blood and lymph in response to invasion by an antigen .
Antibodies are globulin s formed in lymphoid tissue s by B cell s, 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 antitoxin ; reticuloendothelial system .