Any substance that causes the immune system to produce specific antibodies or T cells against it. An antigen may be a foreign substance from the environment (such as chemicals, bacterial or viral proteins, or pollen) or formed within the host's own body. Reactions of antigens with antibodies or T cells serve as a defense against microorganisms and other foreign bodies, but can be detrimental if the immune response is mounted against the “self,” as in autoimmune disorders. Antigen-antibody complexes are used in laboratory tests for detecting the presence of either antigen or antibody to determine a subject's previous exposure to pathogens.