The introversion of the immune response: a hypothesis for T-B interaction

B cells present specific antigen efficiently(1-3) and, like macrophages, internalize antigen before apparently presenting it in association with class II MHC molecules. The association seems to occur when antigen bound to surface immunoglobulin (sIg) is internalized in a vesicle containing class IIMHC molecules(4). sIg is partly degraded in the vesicle and not re-expressed in native form' but class II molecules are re-expressed(6). Class II molecules have little or no polymorphism. How can they associate with all antigens in such a way that T helper cells can recognize the complex? Here Lee Leserman offers a solution to this problem based on the suggestion that T cells recognize the Ig in a complex of associated Ig and class II molecules, rather than the antigen in an antigen-class II complex. The system is antigen-dependent, not because the antigen has to associate with class II molecules, but because antigen is necessary for the internalization of sIg which is required for sIg association with class II.