The interaction of the Facb fragment of rabbit anti-sheep red cell IgG with guinea pig macrophages, and human monocytes and granulocytes.

Yasmeen et al. (J. Immun. 110, 1706-1709, 1973) have previously reported on the binding requirements of the guinea pig peritoneal macrophage Fc receptor. The C gamma 3 domain fragments of human IgG1, in contrast to the C gamma 2 domain fragment, were able to bind to these macrophages, as demonstrated by both direct and indirect rosette tests. We now report that we have been unable to show binding by the C gamma 2-bearing rabbit Facb fragment to either peritoneal or alveolar macrophages of the guinea pig. This evidence is therefore in agreement with the hypothesis proposed by Yasmeen et al. (1973) that the C gamma 2 homology region does not contribute directly to the binding requirements for this cell type. The same protein, rabbit anti-sheep erythrocyte Facb, when coated on sheep erythrocytes, did not form rosettes with human granulocytes, but did form some rosettes with human monocytes.