What is an antigen?

An antigen is a substance that stimulates an immune response. When exposed to an antigen, the body sees it as foreign material and takes steps to neutralize the antigen. Typically, the body accomplishes this by making antibodies, which are meant to defend the body from invasion by potentially dangerous substances. Various medical tests can be used to detect antigens, 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 the generation of antibodies within the immune system. These antibodies can be beneficial, as is the case when the body learns to fight off a virus like measles, or they can be harmful, in the case of allergies. The unique signatures of these antibodies can be identified in medical tests, which can 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, various cells perform specific functions that help the immune system recognize and deal with potential threats to its host. Some of these cells learn to recognize substances that are not from the host organism. When an antigen is identified, these cells alert other cells to the problem and the body takes action.

Various 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 of antigen, as the body realizes that the substance is foreign and potentially dangerous. Transplanted organs and tissue material can also mount an antibody response, since the body does not recognize them as part of the host organism. Because of this problem, people undergoing organ transplants take immunosuppressive drugs designed to limit the antigenic response so that the body does not reject the inserted organ.

Sometimes the body develops an antigenic response to something that is actually not harmful. This is better known as an allergy. In these cases, the body is exposed to a small amount of the antigen, such as from peanut butter, wheat, or a bee sting. Helper cells of the immune system tag the substance, causing the body to produce cells that will counteract the substance if it reappears in the body. 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 that can cause severe discomfort and even death.

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