Dosage is a phenomenon in which, when a gene is present in the homozygous state (double dose), there will be more antigens per red cell than in the heterozygous state (single dose).The effect is exhibited by the corresponding antibody reacting more strongly with cells from homozygous individuals as shown below:




anti-c 2+ 4+
anti-c (weak) 0 * 2+

* false negative with C+c+ red cells

As shown in this example, anti-c reacts more strongly with homozygous (cc ) cells than it does with heterozygous (Cc ) cells. Note: Technically speaking, humans and not red cells are homozygous or heterozygous for genes. However, it is convenient to refer to the red cells as if they were homozygous/heterozygous.

If an antibody is weak, it may give a false negative result with cells from a heterozygous donor.

Because of dosage effect it is important that

  • antibody screen cells be homozygous for antigens that show dosage
  • when patients have a positive antibody screen and crossmatch-compatible donors exist, the antibody should be identified and donors antigen typed before being issued for transfusion

Discussion Question

  1. Which red cell antigens typically show dosage?