Blood group: An inherited feature on the surface of the red blood cells. A series of related blood types constitute a blood group system such as the Rh or the ABO system.

The frequencies of the ABO and Rh blood types vary from population to population. In the US, the most common type is O+ (meaning O in the ABO system and positive in the Rh system) which is present in 37.4% of the population. The frequencies in the US (in descending order) are O+ (37.4%), A+ (35.7%), B+ (8.5%), O- (6.6%), A- (6.3%), AB+ (3.4%), B- (1.5%) and AB- (0.6%).

In 1901 a Viennese pathologist named Karl Landsteiner (1868-1943) published an article entitled "On Agglutination Phenomena of Normal Human Blood," in which he observed that, when blood was transfused from one human to another, the body often clumped the transfused blood cells and rejected the transfusion, sometimes going in shock. In 1909 Landsteiner classified red blood cells into types A, B, AB and O and showed that the body rejects transfusions of a different blood type. After moving to the Rockefeller Institute in New York, Landsteiner received the Nobel Prize in 1930 for his pioneering research in immunology and blood grouping.