The 11-5.2 antibody reacts with the alpha chain of the I-Ak 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 11-5.2 antibody cross-reacts with cells from mice bearing the H-2r haplotype. Reactivity with other H-2 haplotypes (e.g., b, d, f, p, q, s) has not been observed. The 11-5.2 antibody has been reported to block antigen presentation and induce B cell differentiation.