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By Levi Clancy for Student Reader on

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Antigens are foreign bio-organic molecules that interact with B cells (via antibodies) and T cells (via T cell receptors). Blood-borne antigens are concentrated in the spleen; lymph-borne antigens are concentrated in nearby lymph nodes and nodules. Upon detection by the acquired immune response, antigens stimulate production of antigen-specific antibodies. Toxins, invading bacteria and viruses, and the cells of transplanted organs can all function as antigens. Bio-organic chemicals are those based on carbon and the atoms which bond to carbon (hydrogen, oxygen, nitrogen, phosphorous and sulfur -- aka CHONPS). An antigen's configuration of CHONPS is called its antigenic determinant. By ignoring any non-CHONPS chemical, the immune response does not recognize sand, mercury, minerals and other potentially hazardous contaminants. By just recognizing bio-organic antigens, the immune response has evolved to detect only antigens encoded or controlled by genes.

Hapten-CarrierHaptenCarrier Protein
DNP-BGGDinitrophenolBovine gamma globulin
LAC-HGGPhenyllactosideHuman gamma globulin
NIP-KLHS-nitrophenyl acetic acidKeyhole limpet hemocyanin
TNP-BSATrinitrophenylBovine serum albumin

Antigens are any substance that binds specifically to B cell receptors or T cell receptors. There are two types of antigens: immunogens and haptens. An immunogen is a any substance that can elicit an innate or acquired response, and haptens are research-useful small molecules which must be attached to a carrier molecule to elicit a response.

The immune response has two cells that recognize antigens: B cells and T cells. B cells present immunoglobulin (antibody molecule) and T cells present T cell receptor (TCR). The function of these cells is to bind antigens and to remove them from the system. Antibodies and TCRs are specific to the antigenic determinant of a particular antigen. B cells respond to different antigens in different ways (thymus-dependent and thymus-independent) and this is described in the first two paragraphs of B cell activation.

Antigens are difficult for the immune response to detect due to the size of the antigenic universe and because the antigenic universe is constantly changing. The size of the antigenic universe is due to the billions of foreign microbes around. With each microbe containing a few foreign molecules, the number of antigens is inconceivably large. Also, genetic change causes this antigenic universe to constantly change. Bacteria replicate at a fast rate, meaning antigenic drift (change) is too fast for standard mechanisms that deal with diversity.