Blood groups and red cell antigens

Blood type is determined by markers (antigens) that are scattered across the surface of red blood cells (RBCs). These antigens take a variety of different forms: they may be sugars that project above the cell surface, or they may be large proteins that form an important part of the RBC membrane. The presence and absence of these antigens make the blood from different people, different. And there are many types of blood because there are hundreds of antigens.