Abstract
Rat basophilic leukemia (RBL) cells carry two surface glycoprotein molecules named R (or α) and H which, when detergent solubilized, bind to rat IgE-Sepharose. The same two molecules also bind to rat IgG-Sepharose but with a lower affinity. R is a component of the high affinity Fc receptor for IgE. In the present study the inhibition of the binding of R and H to rat IgG-Sepharose by various homologous and heterologous immunoglobulins was used to assess their relative affinities for the two receptor molecules. Ranking the rat immunoglobulins in order of their affinities for the R receptor yielded: IgEx > IgG 2a > IgG 1 > IgG 2b; and for H: IgE > IgG 2b > IgG 1 > IgG 2a. Rat IgG 2c inhibited the binding of both R and H but a precise ranking could not be assigned. Conclusive evidence has been obtained for the Fc specificity of these interactions. The affinities of the mouse IgG subclass/R interactions can be ranked: IgG 1 > IgG 2a > IgG 2b; and for the H receptor: IgG 1 > IgG 2b > IgG 2a. All of the mouse proteins and other heterologous IgGs, such as those of sheep, goat, equine and rabbit origin, interacted considerably more strongly with H than with R. No interaction with mouse IgG 3 could be detected under the conditions tested.
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