Antibodies are Y-shaped gamma globulin proteins which are found on the surface of B cells lymphocytes (white blood cells). They are released into the blood stream and lymph in response to intruding antigens (bacteria, bacterial toxins, fungi, parasites, viruses, and cell proteins). Antibodies are used by the immune system to identify and neutralize these foreign microorganisms by binding specifically to them. B cells lymphocytes are capable of producing one type of antibody, each bearing sites on its membrane which binds to a specific antigen. When this binding takes place, it triggers the B cells lymphocyte to reproduce itself, forming a clone that manufactures vast amounts of its antibody. Antibodies are produced by a kind of white blood cell called a plasmocyte, which originates in the bone marrow as B cells.
The antibody molecule consists of four polypeptide chains; 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.