An antigen is any foreign substance which generates antibodies, provoking an immune response in the organism it has entered. Antigens include bacteria, viruses, foreign blood cells, and the toxins they secrete. When a human body suffers from autoimmune disorders, its own immune system reacts to its own antigens (self-antigens), which are the white blood cells and its toxins with which they would attack bacteria and viruses under normal and healthy circumstances. The term "antigen" is formed from the words "antibody generator". Specially prepared microbial antigens which are used to induce protective antibodies are called vaccines.
An antigen is bound by an antibody at its antigen-binding site, then it is destroyed by the host's phagocytic white blood cells. It is important to bear in mind that antibodies tend to discriminate between the specific molecular structures presented on the surface of the antigen . Antigens are also proteins or polysaccharides from the surface or the inside of a viral or bacterial cell.
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