The rabbit red cell antigen Hg A and anti-Hg A .

The number of Hg sites on rabbit red cells was found to lie between 1.38 and 1.90 × 10 per cell in heterozygotes (HgHg) and between 3.0 and 3.8 × 10 per cell in homozygotes (HgHg). The lowest concentration of IgG anti-Hg which could be detected using a `standard' indirect antiglobulin test was in the range 0.02–0.08 μg/ml. The equilibrium constant of different examples of IgG anti-Hg lay between 0.5 and 1.56 × 101/mole. The ability of different examples of IgG anti-Hg to bind complement varied widely: some examples produced macroscopic haemolysis after 1-hour incubation with rabbit complement; one example produced no haemolysis even when the sensitive Cr-release method was used and failed to sensitize red cells to agglutination by anti-C3. The rate of haemolysis produced by haemolytic anti-Hg was very much less than that produced by haemolytic human anti-A.