Effect of IgM and IgG iso-antibody on red cell clearance.

Production of the iso-antibody anti-Hg was stimulated in rabbits and relatively pure preparations of IgG and IgM anti-Hg were obtained by fractionating plasma on DEAE-cellulose. The effect of these preparations on red cell clearance was assessed by injecting different amounts, followed by Cr-labelled Hg-positive red cells, into previously untransfused Hg-negative rabbits. In rabbits injected with IgG antibody, the clearance of a small dose of red cells (0.01 ml/kg) always followed a single exponential curve even when red cell destruction was relatively slow, but in rabbits injected with small amounts of IgM antibody, two-component curves were observed, some cells being cleared very rapidly and others much more slowly. This difference between the effects of IgG and IgM antibodies had previously been suspected from work with human red cell iso-antibodies. Further experiments suggested that the destruction of only a proportion of transfused incompatible red cells, by low concentrations of IgM antibody, was due partly to `exhaustion' of antibody and partly to heterogeneity of the red cell population with respect to the antigen.