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

The number of Hg(A) sites on rabbit red cells was found to lie between 1.38 and 1.90 × 10(5) per cell in heterozygotes (Hg(A)Hg(F)) and between 3.0 and 3.8 × 10(5) per cell in homozygotes (Hg(A)Hg(A)). The lowest concentration of IgG anti-Hg(A) 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(A) lay between 0.5 and 1.56 × 10(8)1/mole. The ability of different examples of IgG anti-Hg(A) 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 (51)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(A) was very much less than that produced by haemolytic human anti-A.