Table 2-3. Conversion of H substance (antigen) to blood groups.
(In order of most amounts of residual H substance to least amounts.)
(4) The second principal subgroups of A are A1 and A2. RBCs of both react
strongly with anti-A reagents. The serologic distinction between these subgroups is
based upon results obtained using anti-A1 reagents agglutinate A1 but not A2 cells.
Approximately 80 percent of group A RBCa are agglutinated with anti-A1, while the remaining
20 percent of group A RBCs are not agglutinated. Some of the lesser subgroups may,
however, cause problems. Lesser subgroups of A (A3, Am, Ax) produce the A antigen,
but in a much smaller quantity than A1 and A2 individuals. This can cause a problem
when performing cell grouping by being mistaken for a Group O. A second problem
occurs with persons with lesser subgroups who may produce Anti-A1. If transfused with
regular group A blood, they may experience a reaction. Table 2-4 lists the genotypes
that can be responsible for the commonly observed red blood cell ABO phenotypes.
Table 2-4. Genotypic basis for common ABO phenotypes.