Any non-self substance capable of triggering an immune response is known as an antigen. An antigen can be a whole non-self cell, a bacterium, a virus, an MHC marker protein or even a portion of a protein from a foreign organism.
The distinctive markers on antigens that trigger an immune response are called epitopes. When tissues or cells from another individual enter your body carrying such antigenic non-self epitopes, your immune cells react. This explains why transplanted tissues may be rejected as foreign and why antibodies will bind to them.
![Markers of Non-Self](https://webarchive.library.unt.edu/eot2008/20080923084609im_/http://www.nci.nih.gov/images/Documents/02853881-1148-4541-b2b6-e8ecd3e50c6f/cancer3.jpg)
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