Table 2-6. Routine ABO grouping.
2-13. LESS COMMON PHENOTYPES
a. Difference Between A1 and A2.
(1) The most common variation within the standard categories of A, B, AB,
and O is the A2 phenotype. In approximately 20 percent of A and AB persons, the red blood
cells have A2 reactivity. With present-day reagents, there is virtually no difference in the
strength of the agglutination observed with commercial anti-A. Quantitative evaluation,
however, reveals that A2 cells have only one-third to one-quarter the number of A
antigenic sites that A1 cells have. A2 cells have substantially more H reactivity than A1
cells. A2 individuals who have the secretor gene have A substance in their saliva.
(2) It appears that qualitative as well as quantitative difference exists
between A1 and A2 cells. A few A2 individuals, and as many as 20A2B individuals, have
anti-A1 activity in their serum. This reacts, usually weakly, with A1 or A1B cells and not
with A2 or A2B cells. Similarly, the anti-A in serum from a group B individual contains
some antibodies that react with both A1 and A2 cells, and some which react only with A1
cells. Absorbing group B serum with A2 cells leaves anti-A1 activity in the supernatant.
Saline extracts of seeds of Dolichos biflorus agglutinate A1 cells, but not A2 cells.
b. Weak Subgroups of A12,13.
(1) Much rarer than A2 are subgroups of A with still fewer A sites on the red
blood cells. These result from the action of mutant genes, comprising less than 1 percent of
the total pool of A genes. Of these rare types, the most common (perhaps 0.1 percent or less
of group A) is A3, in which the red blood cells characteristically give a mixed-field
agglutination pattern when tested with anti-A. Small clumps of agglutinated cells are
present among large numbers of cells which adsorb the antibody to their surface but are
not agglutinated by it. A3 cells are not agglutinated by Dolichos biflorus extract or anti-
A1. They react more strongly with anti-H than A2 cells do. A few A3 persons have anti-
A1 in their serum, but this is rare. ABH secretors who are A3 have A substance in their