The distribution of the Diego factor has been studied in various Mongoloid, Caucasian and Negroid populations. In six South American Indian stocks, one Canadian Indian stock and two groups of Asiatic Mongoloids it was present in high incidence. In Caucasians tested, negative results were found. In two groups of Venezuelan Negroes with probable Indian admixture the factor was also present.

The Diego antigen was inherited in single dose (heterozygous) in 41 families studied and could be followed through several generations with no sex-linkage.

The first samples of anti-Diego serum reacted only in the indirect anti-globulin test to a titer between 1:256 and 1:512. The enzymes papain and trypsin gave no advantage. It is more active at room temperature (25 C. and 37 C.) than at cold temperature (4 C. and 18 C.). The most suitable pH range is from 5.0 to 8.0.

The Diego factor can be considered as part of a new blood group system (the Diego system) which could be classified as the tenth firmly established blood system.

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