Updated June 10, 2015.

Definition: An antigen is a molecule that may be present on the surface of a red blood cell and other cells in the body. The antigen is recognized by the immune system, and if the antigen is recognized as an foreign body, the immune system will work to reject the foreign material by sending antibodies.

When the immune system detects the antigen of an intruder, such as the common cold, it triggers the body to fight it off.

When the wrong blood type is detected, the body will also fight off the intruder, but the process can be deadly. For this reason, blood from an O- donor, which has no antigens, is used when the recipients blood type is unknown.

Alternate Spellings: antigen, antigens,

Common Misspellings: antagen, antijen, antijin,

Examples: O- blood has no antigens, which makes it safe to donate to any recipient.