From Wikipedia:

Antigen

In immunology, an antigen is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. The term antigen originally referred to a substance that is an antibody generator. Antigens can be proteins, peptides, polysaccharides, lipids, or nucleic acids... Read More