A foreign substance, such as a transplanted organ or tissue, that triggers the body to reject it (destroy it.)
(Organdonor.gov April 2020)
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, pollen, or foreign tissues. An antigen may also be formed within the body, as with bacterial toxins.
(OPTN Glossary May 2021)