antigen
Meaning of antigen: An antigen is any substance the immune system recognizes as foreign, prompting it to mount a targeted immune response, including antibody production.
Definition of antigen
An antigen is any substance capable of triggering a specific response from the immune system. Antigens are commonly found on the surface of bacteria, viruses, fungi, and other foreign organisms, but they can also appear on transplanted tissue, pollen, and even the body's own cells when those cells are abnormal, such as in cancer.
When the immune system encounters an antigen it identifies as foreign, specialized immune cells produce antibodies, proteins shaped to bind that particular antigen, or activate other immune cells that target it directly. This recognition allows the immune system to distinguish threats from the body's own healthy tissue.
Antigens are also central to laboratory testing and vaccination, since detecting a specific antigen can confirm infection with a particular organism, and vaccines work in part by exposing the immune system to a harmless form of an antigen so the body can respond more quickly if exposed again later.
Used in a sentence
Laboratory testing detected a viral antigen in the sample, confirming active infection.
Good to know
An antigen triggers the immune response, while the antibody produced against it is a distinct molecule shaped to recognize and bind that specific antigen, a distinction that matters when interpreting antigen and antibody test results together.
Word origin
Where “antigen” comes from
Formed from anti-, Greek for against, and the suffix -gen, from genes, meaning producing, since an antigen is a substance that generates an immune response against it.