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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Antibody /an·ti·body/ an immunoglobulin molecule that reacts with a specific antigen that induced its synthesis and with similar molecules; [more]Antibody - Community and Resources

Noun
1. any of a large variety of proteins normally present in the body or produced in response to an antigen which it neutralizes, thus producing an immune response
(hypernym) protein
(hyponym) autoantibody
(part-meronym) active site

noun
substance which is naturally present in the body and which attacks foreign substances (such as bacteria); tests showed that he was antibody-positive