Large molecules, anything that stimulate a specific immune response.

In immunology, an antigen is the substance that binds specifically to the respective antibody. The term antigen originally came fromANTIbody GENerator.

The antigen may originate from within the body or from the external environment. "Self" antigens are usually well tolerated by the immune system, which has been educated to non-reactivity against the structures present inside the body under the physiological conditions. "Non-self" antigens can be identified as invaders from the outside world or modified/harmful substances

References:

Wikipedia: http://en.wikipedia.org/wiki/Antigens