Dosage Effect Dosage is a phenomenon in which red cells from homozygous people possess more antigens per red cell than do cells from heterozygous people. The effect is exhibited by the corresponding antibodies reacting more strongly with cells from homozygous people (Table 5-2).
*NA = not applicable
As shown in Table 5-2, anti-c reacts more strongly with homozygous (cc) cells than it does with heterozygous (Cc) cells, as does anti-e. Note that if the antibody is weak, it may give a false negative result with heterozygous cells. This is why it is important that antibody screen cells be homozygous for antigens that show dosage.
In the Rh system, C, c, E and e antigens show dosage. The D antigen does not show dosage in the usual sense, because the d antigen does not exist. However, the amount of D antigen does vary significantly from person to person according to the person's Rh genotype. It is known that cells with the R1r genotype have a relatively small number of D antigens per cell (10,000 - 15,000). In contrast, R2R2 cells have a relatively large number of D antigens (16,000 - 33,500) as do R1R2 cells (23,000 - 31,000).
We can use this information in selecting positive antisera controls when antigen typing. Because positive antisera controls should have weakly reactive antigens (to test the potency of the antisera), heterozygous cells are chosen. In the case of anti-D antisera, R1r cells are appropriate positive control cells, but R2R2 cells are not.