What are antigens?

Parts of molecules: Anything that one creature's immune system recognizes (rightly or wrongly) as foreign and reacts against can be considered an antigen. Usually these are sequences within proteins. We pathologists use antibodies against normal components of human tissue made by other animals to tell what's in cells we are examining to tell what they are; blood bankers must choose donor units with the right antigens.

Antigens are...: An antigen is any substance that causes your immune system to produce antibodies against it. It may be a foreign substance from the environment such as bacteria, viruses, pollen or chemicals. An antigen may also be formed within the body, as with bacterial toxins.