A specific type of white blood cell — the B lymphocytes — produce antibodies as part of the immune system. When lymphocytes detect the presence of a destructive substance — an antigen — the immune system reacts by forming antibodies. Each type of antibody is effective against one specific antigen.
Antibodies are specific types of proteins. They typically defend against bacteria, fungi, parasites and viruses. Sometimes, however, the immune system sees healthy tissue as an antigen and sends out antibodies. This results in an autoimmune disorder, when the body is essentially attacking itself. Examples include celiac disease, an intestinal inflammation; Grave's disease, which causes an overactive thyroid; and rheumatoid arthritis, which is characterized by chronic joint and tissue inflammation.