a substance recognized as foreign by the body; stimulates antibody production and a T cell response.
a foreign substance in the body (a bacterium, virus, or protein, for example) that can cause disease — and whose presence triggers an immune response (the formation of antibodies).
A substance capable of inducing a specific immune response in the body, by binding to a specific antibody; can be a property of bacteria, viruses, other foreign proteins, or even host tissue cells.
A substance that stimulates an immune response. The immune system recognizes these substances as being foreign, and produces antibodies to "fight" them. This antigen/antibody response is an important part of immunity.