The 14-4-4S antibody reacts with the I-Ek MHC class II alloantigen. These class II molecules are expressed on antigen presenting cells (including B cells) and a subset of T cells from H-2k bearing mice and are involved in antigen presentation to T cells expressing CD3/TCR and CD4 proteins. The 14-4-4S antibody cross-reacts with cells from mice of the H-2d, H-2p, and H-2r haplotypes and the rat MHC class II antigen RT1D. Cells from mice of the H-2b, H-2f, H-2q, and H-2s haplotypes do not express I-E antigen. The 14-4-4S antibody has been reported to block antigen presentation and induce differentiation of mouse cells bearing I-Ek.