I-region genes are expressed on T and B lymphocytes. Studies of the mixed lymphocyte reaction (MLR).

Unidirectional mixed lymphocyte reactions (MLR) were performed between mouse strains differing for various segments within the H-2 complex. Thymocytes and purified lymph node T cells and B cells were used as stimulator cells. In three of five combinations studied, differing only within the I region, both T and B cells stimulated in the MLR. This suggests that the region codes for both T- and B-cell surface structures. However, if the difference was restricted to one I subregion (I-C), only T cells stimulated. This finding suggests that some of the I-region genes may be expressed either in T or in B cells.