Antigen

From ArticleWorld

An antigen is any molecular structure that elicits an immune response in a host, particularly causing the host to produce an antibody that is specific to the antigen. An antigenic molecule is generally attached to a pathogenic organism such as a bacterium or a virus. It can be made of proteins, polysaccharides (sugars) or some combination of both.

Origins

Antibodies come from several different origins. Exogenous antigens are those that enter the body from the outside. These antigens are swallowed by immune cells and then processed into fragments. The antigens are presented to T-helper cells that become activated and release cytokines. Cytokines then activate the pathogen-eating cells and the antibody-producing cells.

Endogenous antigens are a result of normal cell metabolism or occur on the surface of a cell that has been infected by a pathogen. These antigens appear on the outside of the cell and, because they are seen as foreign, the cells undergo lysis (destruction) by toxins secreted by T-cells.

Tumor antigens are found on the surface of a cancerous cell as part of the mutation a cancer cell undergoes. If cytotoxic T-cells see the tumor antigen as foreign, the tumor cell is then destroyed.

Types

There are three specific types of antigens. An immunogen is a molecular substance that that provokes an immune response in the host. A tolerogen is an antigen that is recognized by the immune system but is not generate an immune response. An allergen is a specific antigen that triggers an allergic reaction in the host, usually entering the host by means of inhalation, injection or skin contact.