Interactions between immunoglobulins and I region antigens on the B lymphocyte membrane

Modulation and co-modulation of membrane-bound IgM and IgD and of I region-encoded antigens (I-A and I-E) were studied by immunofluorescence with the use of monoclonal antibodies. The two sets of molecules displayed different ability to be redistributed, by divalent ligands: immunoglobulins could be easily redistributed, while this was not the case for I-A and I-E molecules. After short periods of activation, however, this property was found to change drastically: I-region antigens were redistributed into polar caps, and immunoglobulins were modulated without clear polar redistribution. Cross-linking and removal of one of the molecules always resulted in the modulation of at least one other, often two membrane molecules. Concomitantly, an additional molecule was expressed in higher amount. The results are compatible with the existence of specific interactions at the level of the B cell membrane, involving the four molecules in the order I-A----IgM----I-E----IgD. Such interactions, probably reversible in the unperturbed state, are likely to change in affinity when one molecule interacts with a ligand. The possible relevance of membrane interactions between immunoglobulin and class II antigens in the regulation of the B cell is discussed.