An antigen is any substance that can stimulate the immune system to produce antibodies against it
. Antigens can be foreign substances such as chemicals, bacteria, viruses, pollen, or allergens, as well as substances produced by the body itself
. Antigens can be proteins, peptides, polysaccharides, lipids, or nucleic acids
. They exist on the surface of cells, viruses, bacteria, fungi, parasites, and tumor cells
. Antigens are recognized by antigen receptors, including antibodies and T-cell receptors
. Upon exposure to an antigen, only the lymphocytes that recognize that antigen are activated and expanded, a process known as clonal selection
. Antibodies are very specific to the antigens they recognize and destroy, and they fit onto the antigen like a key to a lock
. There are several types of antigens, categorized by where they come from, including exogenous antigens, endogenous antigens, autoantigens, and tumor antigens
