classification of human blood according to the properties conferred by the presence of an antigen designated Di. There are 21 known Diego antigens; however, the determination of an individual's Diego blood type is based on the antigens denoted Di^a (identified in 1955) and Di^b (identified in 1967). The Diego blood group system is associated with a gene known as SLC4A1. This gene encodes a substance called band 3 protein, which is expressed on the surface of red blood cells (erythrocyte) and plays a central role in mediating the transport of carbon dioxide in the blood. While mutations (mutation) in SLC4A1 can give rise to diseases such as hereditary ovalocytosis (a disease in which red blood cells are oval-shaped, not round), a number of other mutations result in the production of Diego antigens.

The frequency of the Di^a antigen is found in more than 35 percent of South American Indians and about 12 percent of people of Chinese and Japanese descent. Diego incompatibility of mother and fetus can cause erythroblastosis fetalis, in which an infant's red blood cells are destroyed by antibodies produced by the mother.