What is an antigen?
An antigen is a substance that causes an immune response, activating the lymphocytes.
An antigen is a substance on the surface of cells that causes an immune response, activating the lymphocytes which produce antibodies specific to the type of antigen. Antibodies bind to the antigen, either neutralizing it as a threat or flagging it for macrophages to destroy.
In the image below, the macrophage is the large purple cell:
Antigens may be external to the body (heteroantigens), such as substances in snake venom or produced by bacteria, or they may be produced internally (self-antigens), such as proteins encoded by cancer cells.