A substance that when introduced into the body stimulates the production of an antibody. Antigens include toxins, bacteria, foreign blood cells, and the cells of transplanted organs.
The molecule which determines your blood type is an antigen, because outside of you it might be considered ‘foreign’ by a person of another blood group who carried an antibody against it. It is important to remember that the particular sugars that make a person blood type A, or B, or O are not limited to humans. Not only can they be found on other higher animals, such as mammals, they are also found as antigens on very simple life forms as well, such as fungi and bacteria.
For example, a recent study showed that of 833 fungi harvested from 1977 to 1994, 422 extracts (47.8%) produced agglutination of human red blood cells, equally distributed against type O, A and B cells. Obviously, these fungi are not directly attacking human red cells, but rather are using their own lectins to attach and infect seeds or other microbes which possess some ‘ABO blood type’ activity of their own.
- Furukawa K, Ying R, Nakajima T, Matsuki T. Hemagglutinins in fungus extracts and their blood group specificity. Exp Clin Immunogenet 1995;12(4):223-31
The ABO antigens are chains of carbohydrate molecules; the type A antigen is N-acetylgalactosamine; the B antigen is galactose and the O or H antigen is fucose.