The types of proteins, glycoproteins and glycolipids found (or expressed) on the surface of red blood cells define blood types. In addition, blood types, or at least the genes responsible for them, are inherited. Karl Landsteiner described the original blood types--A, B and O--in 1900 and doctors now recognize 23 blood group systems with hundreds of different "types." Many of the blood type antigens (and the antibodies that react with them) have been discovered as a result of transfusion incompatibilities. Some of these molecules have additional functions that are at least as important as conferring compatibility (of lack of compatibility) on transfusion recipients, however. The advent of sophisticated biochemistry and molecular biology has helped to characterize a number of these entities. Although it appears that the majority of the molecules are not essential for red cell function, some have specific functions on the red cell membrane such as allowing substances to enter and exit the red cell or binding certain substances to the cell surface.For some blood types, evolution and environmental selective pressures are clearly important for their persistence. For example, the Duffy blood type includes a receptor that allows certain types of malarial parasites to enter the red cell. Thus, in some malarial areas of Africa, populations with Duffy-negative blood types have a distinct survival advantage because absence of the Duffy antigen provides a measure of protection against malaria. The percentage of people lacking the Duffy antigen is much higher in these locations than in areas not endemic for malaria.