Rh ANTIBODIES AND THE PROZONE PHENOMENON BY

In 1942 Taylor, Race, Prior, and Ikin reported an anti-Rh serum which failed to agglutinate Rhpositive cells in saline suspension except when diluted. They thought that this prozone was due to antigen and antibody not being present in optimal proportions, and that the mechanism was similar to that of the zones that occur in precipitation reactions. Wiener (1944) described incomplete Rh antibodies and their blocking effects, and considered that this type of antibody might be responsible for the prozones seen with Rh antisera. Subsequently, various other workers observed prozones with anti-human globulin sera, and Hubinont (1951) considered that these were due to lack of optimal proportions between antigen and antibody. Van Loghem (1950) also investigated the prozones of antiglobulin sera and considered that they might be due either to the presence of blocking antibodies or to overdosage by excessive amounts of agglutinin, i.e., lack of optimal proportions. Van Loghem, Kresner, Coombs, and Fulton Roberts (1950) considered these two alternative explanations further, and described experiments to show that excess of agglutinin was the cause of the prozone in the particular antiglobulin reagent which they were studying. They suggested that, when agglutinin is present in high concentration, all the available antigen sites on the red cell surface become occupied by molecules of agglutinin, so that none are free to combine with molecules of agglutinin already attached to other cells, and the lattice necessary for agglutination cannot be formed. This was demonstrated by the fact that washed cells from the prozone would agglutinate when mixed with a further supply of cells bearing free antigen receptors. Cells from the prozone also agglutinated on the addition of suitable concentrations of human y globulin. The conclusion drawn from these experiments, that the prozone, in this particular case at any rate, was caused by excessive amounts of agglutinin and not by incomplete antibody, appears at first sight to be irrefutable. Nevertheless, certain considerations led us to suppose that this conclusion might be incorrect and that the same phenomenon might occur even though the cause of the prozone was incomplete antibody. The first experiment described below shows that that is so, at any rate in the case of Rh antibodies. The second experiment shows that in some cases even cells from the first negative tube of a titration will agglutinate on the addition of more cells with free receptors.