antigen

antigen - A substance that can trigger an immune response, resulting in production of an antibody as part of the body’s defence against infection and disease.

Many antigens are foreign proteins (not found naturally in the body) such as parts of microorganisms and toxins or tissues from another person that have been used in organ transplants.

Sometimes, harmless substances (pollen, for example) are misidentified by the immune system as potentially harmful antigens, which results in an allergic response (see allergy).