An antigen is a substance which stimulates an immune response. When exposed to an antigen, the body views it as foreign material, and takes steps to neutralize it. Typically, the body accomplishes this by making antibodies, which are intended to defend the body from invasion by potentially dangerous substances. Several medical tests can be used to test for the substances, to determine whether or not someone has been exposed to a disease or toxin.
The term “antigen” comes from the understanding that many foreign substances stimulate antibody generation within the immune system. These antibodies can be beneficial, as is the case when the body learns to fight a virus like measles, or they can be harmful, in the instance of allergies. The unique signatures of these antibodies can be identified in medical testing, which may be used to determine why a patient is exhibiting a set of symptoms.
Most animals have what is known as an adaptive immune system. Within the immune system, a number of cells serve specific functions which help the immune system recognize and deal with potential threats to its host. Some of these cells learn to recognize substances which are not from the host organism. When an antigen is identified, these cells alert other cells to the problem, and the body takes action.
A number of things can be sources of antigens. Humans can inhale or ingest bacteria and viruses from other organisms, for example. A toxin can also be a source, as the body realizes the substance is foreign and potentially dangerous. Transplanted organs and tissue material can also generate an antibody response, since the body does not recognize them as being part of the host organism. Because of this issue, people who undergo organ transplants take immunosuppressive drugs which are designed to limit the response so the body does not reject the introduced organ.
Sometimes, the body develops an antigen response to something which is not actually harmful. This is better known as an allergy. In these instances, the body is exposed to a small amount of the antigen, such as peanut butter, wheat, or a bee sting. The helper cells in the immune system tag the substance, triggering the body to produce cells which will counter the substance if it appears in the body again. When the unwitting human host eats peanuts, nibbles on a slice of toast, or is stung by a bee again, the body launches an antibody response which can cause severe discomfort and sometimes even death.