Diego antigen system
The Diego Antigen (or Blood Group) System comprises 21 rare blood factors, any of which is carried on the band 3 protein, coded for by the gene SLC4A1 (Solute carrier family 4, Anion exchanger, member 1), located on human chromosome 17. The band 3 protein functions in the transport of chlorine ions and carbonate. The resulting factor of each SLC4A1 allele is classified as a separate blood group.
Diego antigens are only found (or in some cases, only not found) in populations of Aboriginal Americans (in both North and South America) and the Mongolic peoples of East, Southeast, North-Central and Northeast Asia. Incidence of the factors is not diminished in ethnically mixed populations. Indeed, the first two Diego factors were found in people of mixed European and Aboriginal American ancestry.
History and occurrence 
The Diego and Wright antigens were thought to be separate systems until 1992, when it was realised they were part of one system. The labeling of individual factors, however, has not changed, with the former Wright factors still signed 'Wr' while the rest of the Diego factors are signed 'Di'.
The first component of the system, Diegoa, was discovered in Venezuela in 1955, when one Sra. (Mrs.) Diego gave birth to a child who died from Hemolytic disease of the newborn. Rh system mismatch was initially suspected, but Mr. and Mrs. Diego were both RhD positive so it could not have been that. Mr. Diego, however, had some aboriginal South American ancestry that Mrs. Diego did not, and that fact ultimately led to the discovery of Diegoa.
Diegob was discovered under similar circumstances in Mexico in 1967. In this case, however, the problem was not the father being Dib+ but the mother was Dib-. The occurrence of Dib- practically does not exist in most ethnic populations, and is exceptionally rare (about 0.001%) even among populations in which Dia+ is found.
In 1992, the two Wright antigens, Wra and Wrb, were reclassified as part of the Diego system, which now contains 21 known factors.
The two most important factors in the system are Dia/Dib and Wra/Wrb ('Di' for "Diego" and 'Wr' for "Wright"). Each pair contains a low-incidence antigen and an antithetical high-incidence determinant, respectively. Most ethnic populations of the world, for example, present as Dia-/Dib+.
In particular, Dia+ has an insignificant occurrence (0.01%) in populations of Indo-European, West and Southwest Asian, African, and Australasian ancestry. The occurrence is comparatively significant in Southeast, East, North-Central and Northeast Asian, presenting in 5% to 15% of these populations; and more significant in Aboriginal North and South American peoples, where occurrence ranges between 0% and 36%, with higher percentages of occurrence more often found in South American aboriginal populations than North American. There is, however, no apparent pattern to Dia+ distribution in either American continent, nor in Asia.
One constant is that Dia+ may be found in any person with ancestry from one of the Mongolic-Aboriginal American groups in which the factor occurs. Thus, when an American of Polish extraction presented as Dia+, research of his ethno-cultural ancestry traced his family to southwestern Poland and a population of mixed Polish and Tatar ancestry. He inherited the gene for the Diego A factor through the latter side of his ancestry. The cultural distribution of the blood group lends it some importance to the science of anthropology.
- Laura Dean. "The Diego Blood Group". Blood Groups and Red Cell Antigens. Bethesda: National Center for Biotechnology Information; 2005 [cited November 4, 2011]. PDF available.
- Mallen, MS; Arias, T (1959). "Inheritance of Diego blood group in Mexican Indians". Science 130 (3368): 164–5. doi:10.1126/science.130.3368.164-a. PMID 13668547.
- Layrisse, M; Wilbert, J (1961). "Absence of the Diego antigen, a genetic characteristic of early immigrants to South America". Science 134 (3485): 1077–8. doi:10.1126/science.134.3485.1077. PMID 14463057.