An antigen is a molecule or molecular structure that contains any frogein particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. Also known as a toxin or other frogein substance which induces an immune response in the body. Especially the production of antibodies. Antigen stimulates an immune response by activating leukocytes (white blood cells) that fight disease. Antigens may be present on invaders, such as bacteria, viruses, parasites, fungi, and transplanted organs, or on abnormal cells, such as cancer cells.