RESUMEN
The in vitro interaction between monocytes and erythrocytes sensitized with non-complement binding IgG antibodies (i.e. the Rh antibody anti-D:EAIgG anti-D) is completely inhibited by low concentrations of IgG (E.G. 30--100 microgram/ml). However, the interaction between monocytes and erythrocytes sensitized with IgG anti-A (EAIgG anti-A) is not inhibited by IgG. The findings presented in this paper indicate that this difference is probably due to the difference in the number of IgG antibody molecules per EAIgG. Thus, the higher the number of IgG antibody molecules per EAIgG, the less the interaction between EAIgG and monocytes is inhibited by IgG. A second factor which proved to have a strong influence on inhibition by IgG was the number of EAIgG per monocyte. When the number of EAIgG per monocyte was increased from 1 to 32, the percentage of inhibition by a fixed amount of IgG (50 microgram/ml) decreased significantly. This in vitro effect is only evident when relatively weakly sensitized erythrocytes are used and, in vivo, destruction of these weakly sensitized red cells (e.g. EAIgG anti-D) is confined to the spleen. Since a considerable haemoconcentration occurs in this organ, it is conceivable that a high EAIgG:macrophage ratio is accomplished. The latter data are an indication that this high ratio may allow interaction between weakly sensitized erythrocytes and splenic macrophages despite the presence, in vivo, of a high concentration of IgG, and that, in this way, in the spleen, the inhibitory effect of IgG is overcome.