Abstract
The secretor gene, which determines secretion of the ABH blood group antigens into saliva and other fluids, has widely varying frequencies in different healthy populations. Its frequency in patients suffering from certain diseases also differs from that in healthy persons. These observations suggest that frequencies of the gene are to some extent the results of natural selection. One means of selection is through haemolytic disease of the newborn. Women, mostly of group O, become immunized to the A or B antigen of a fetus, and this or subsequent fetuses are at risk from haemolytic disease of the newborn. Affected fetuses are preponderantly secretors; this is probably a genetic consequence of all or most immunizing fetuses being secretors. Selection against the secretor factor as well as against blood groups A and B is thus liable to occur. Similar processes of selection seem to operate through fetal loss by early abortion. It is suggested that counter-selection favouring secretors may result from certain epidemic infectious diseases.
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