What is an Antigen?

Taken off the net (which I find appropriate):


A substance capable of inducing an immune response. Exposure to this substance results in production of a specific antibody or a sensitized lymphocyte that in turn interacts with the antigen. In some autoimmune diseases, the body's own tissues may act as antigens.

Usually reference is made to proteins from foreign bodies (virus, bacteria etc) but (keep it at the back of your mind that) proteins of one self can be considered presented as antigens too, particularly during the development of our immune system so that our immune cells are primed to recognise our cells later on.