Immunoglobulin Binding by Mouse Intestinal Epithelial Cell Receptors¹ Free

The IgG receptors involved in transporting maternal antibodies to the blood stream of the young mouse were studied by feeding internally labeled mouse myeloma proteins and enzymatically generated specific fragments of these proteins. The data obtained from competition experiments showed the following: 1) intestinal receptors have a single specificity which is directed against IgG Fc determinants; 2) IgG subclass proteins differ in their affinities for the receptors and thus have similar but not identical Fc recognition sites; 3) the order of affinities of the IgG subclass proteins for the receptors correlates with the relative concentrations of the four subclass proteins in mouse milk; and 4) several subfractions of the Fc fragment were inactive in binding to the receptors. In many of their properties the intestinal receptors resemble the Fc receptors on other cell types, but they are apparently unique in their ability to bind tightly non-aggregated IgG molecules.