An antibody is a protein produced by the body's immune system when it detects harmful substances, called antigens. Examples of antigens include microorganisms (bacteria, fungi, parasites, and viruses) and chemicals.
Antibodies may be produced when the immune system mistakenly considers healthy tissue a harmful substance. This is called an autoimmune disorder.
Each type of antibody is unique and defends the body against one specific type of antigen.
Abbas AK, Litchman AH, Pillai S. Antibodies and antigens. In: Abbas AK, Litchman AH, Pillai S, eds. Cellular and Molecular Immunology. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 5.
Male D, Brostoff J, Roth DB, Roitt IM. Antibodies. In: Male D, Brostoff J, Roth DB, Roitt IM, eds. Immunology. 8th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 3.