An antigen is any substance that the body regards as foreign and which
therefore triggers a response by the immune
system. Antigens are usually proteins and are commonly found on the surface of infectious organisms (including
some bacteria and viruses),
transfused blood cells, and organ transplants. The presence of an antigen
triggers the production of an antibody,
which reacts specifically with the antigen and either neutralizes it, causes
it to destroy itself, or attracts leukocytes to carry out the destruction.
When an antigen enters the blood (A) it causes a proliferation of lymphocytes (yellow) with specific membrane-bound antibodies (B). These remain in the bloodstream for prolonged periods. If the same antigen (C) attacks again, the antibodies encounter the invaders and neutralize them.