Subgroups of A

Subgroups of A are phenotypes that differ from others of the same ABO group with respect to the amount of A antigen carried on red blood cells(RBCs), and, in secretors , present in the saliva. The two principal subgroups of A are A1 and A2. RBCs of both react strongly with anti- A reagents in direct agglutination tests. The serologic distinction between A1, and A2 is based on results obtained in tests with reagent anti-A1, prepared from group B human serum or the lectin of Dolichos biflorus seeds. Under prescribed testing conditions, anti-A1, reagents agglutinate A1, but not A2 RBCs. The RBCs of approximately 80% of group A or group AB persons are agglutinated by anti-A1, and, thus are classified as A1, or A1B. The remaining 20% whose RBCs are agglutinated by anti-A, but not by anti-A1, are called A2, or A2B. Anti-A1 occurs in the serum of 1% to 8% of A2 persons and 22% to 35% of A2B persons. Anti-A1 can cause discrepancies in ABO testing and incompatibilities in crossmatches with A1, or A1B RBCs. It is considered to be clinically insignificant unless it reacts at 37 C. It is Studies to determine the relative proportion of A1 and A2 in different populations have been carried out. It is a striking fact that the subtype A2 does not seem to occur in the Australian aborigines, in China, Japan, in the Native Americans, or in the natives of the islands of the Pacific, in so far as can be determined from the somewhat sketchy studies made up to the present time. Simmons et al. remark that these observations indicate that the subgroups of A will become a point of greater importance than heretofore in the study of races. Contrast, for example, the relatively high proportion of A2 in Negroes with its total absence in Papuans. not necessary to test group A RBCs with anti A1, to confirm their subgroup status except when working with samples from people whose sera contain anti-A1.