1. An antigen of one's own cells or cell products.
2. A "self antigen"; any tissue constituent that evokes an immune response to the host's tissues.
3. A type of antigen present in the tissues of an organism which causes the development of corresponding antibodies.

An antigen is a harmful substance that causes a person's body to produce antibodies to fight the body itself.

Antigens include toxins (poisons), bacteria, foreign blood cells, and the cells of transplanted organs.