Antibody: An immunoglobulin, a specialized immune protein, produced because of the introduction of an antigen into the body, and which possesses the remarkable ability to combine with the very antigen that triggered its production.
The production of antibodies is a major function of the immune system and is carried out by a type of white blood cell called a B cell (B lymphocyte). Antibodies can be triggered by and directed at foreign proteins, microorganisms, or toxins. Some antibodies are autoantibodies and home in against our own tissues.
The term "antibody" dates to 1901. Prior to that time, an "antibody" referred to any of a host of different substances that served as "bodies" (foot soldiers) in the fight against infection and its ill effects.