Antigens

classification of blood based on antibodies and antigens on red blood cell surfaces

thumb|255px|An illustration that shows how antigens induce the immune system response by interacting with an [[antibody that matches the molecular structure of an antigen]]

classification in immunology

thumb|The genome organisation of HBV; the genes overlap. ORF S, in green, encodes HBsAg. thumb|HBsAg under a transmission electron microscope: the protein self assembles into [[virus-like particles]] HBsAg (also known as the Australia antigen) is the surface antigen of the hepatitis B virus (HBV). Its presence in blood indicates existing hepatitis B infection.

Alloimmunity (sometimes called isoimmunity) is an immune response to nonself antigens from members of the same species, which are called alloantigens or isoantigens. Two major types of alloantigens are blood group antigens and histocompatibility antigens. In alloimmunity, the body creates antibodies (called alloantibodies) against the alloantigens, attacking transfused blood, allotransplanted tissue, and even the fetus in some cases. Alloimmune (isoimmune) response results in graft rejection, which is manifested as deterioration or complete loss of graft function. In contrast, autoimmunity is

processes involved in the biological strategy of changing antigenic determinants on the surface that are exposed to another organism's immune system