Antigens Antigens A blood type, or group, is based upon the presence or absence of certain substances called antigens on the surface of the red blood cells.
Antigens are substances that stimulate cells to produce antibodies .
Antibodies are proteins that are produced to help cells to counter substances foreign to the body, such as antigens, by combining with them in the blood to cause agglutination , or clumping. Antibodies in this way are able to inactivate a foreign substance that threatens the well-being of the body. The most familiar types of antigens are A , B , and O , constituting the ABO system , discovered in 1900 by Karl Landsteiner. The ABO system is an example of a trait which is inherited by multiple alleles. Since blood from Group O individuals contains neither antigens, Group O individuals are called universal donors , and since the serum from AB individuals contains neither antibody, AB individuals are called universal recipients .