Antibody

n. protein in the blood which reacts to infectious agents

Antibodies (also known as immunoglobulins) are proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. They are made of a few basic structural units called chains; each antibody has two large heavy chains and two small light chains. Antibodies are produced by a kind of white blood cell called a B cell. There are several different types of antibody heavy chain, and several different kinds of antibodies, which are grouped into different isotypes based on which heavy chain they possess. Five different antibody isotypes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.
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noun
substance which is naturally present in the body and which attacks foreign substances (such as bacteria); tests showed that he was antibody-positive

[AN-tih-BAH-dee]
A type of protein made by plasma cells (a type of white blood cell) in response to an antigen (foreign substance). Each antibody can bind to only one specific antigen. The purpose of this binding is to help destroy the antigen. Antibodies can work in several ways, depending on the nature of the antigen. Some antibodies destroy antigens directly. Others make it easier for white blood cells to destroy the antigen. 

Treatment with an antibody, a substance that can directly kill specific tumor cells or stimulate the immune system to kill tumor cells. 

Any of various proteins that mark foreign material in the body, such as bacteria and viruses, for removal or destruction by other components of the immune system.