A substance or molecular structure that can be specifically recognized by components of the immune system, such as antibodies or antigen receptors on immune cells.
Antigens can come from infectious agents, such as viruses, bacteria, fungi, and parasites, or from other sources. They may also be molecules from the body’s own tissues, particularly in autoimmune disease.
Recognition of an antigen can trigger or contribute to an immune response. An antigen is not necessarily harmful itself; it simply has features that the immune system can specifically recognize.
QUICK FACT
An antigen is not synonymous with “germ.” A germ may contain many different antigens, and some antigens are not associated with infectious organisms at all.
COMMONLY CONFUSED WITH
Antibody: An antigen is a substance or molecular target recognized by the immune system; an antibody is an immune protein that specifically binds to a particular target.
SEE ALSO