blood group
a category of immunologically distinct, genetically determined traits based on the presence or absence of certain antigens on the surface of red blood cells. The most commonly used method of categorization is the ABO system, in which the basic categories are A, B, AB, and O. There are more than 20 other blood group systems, including the rhesus system (see Rh factor). The major initial work on blood groups and on the nature of blood transfusion was done at the beginning of the 20th century by Austrian physician Karl Landsteiner (1868–1943), who received a Nobel Prize for this research in 1930. Also called blood type.