Antigen
* An antigen is any substance that causes your immune system to produce antibodies against it.

* An antigen may be a foreign substance from the environment such as chemicals, bacteria, viruses, or pollen.

* An antigen may also be formed within the body, as with bacterial toxins or tissue cells.

* An antigen is a substance that can provoke an immune response. Typically antigens are substances not usually found in the body.

Antigen