The M5/114.15.2 (M5/114) antibody reacts with an extracellular epitope on the mouse major histocompatibility complex class II receptor (MHC-II). MHC-II is a heterodimeric transmembrane glycoprotein comprising an α subunit associated non-covalently with a β subunit. In mice, MHC-II complexes are designated I-A and I-E and comprise Aα/Aβ and Eα/Eβ heterodimers, respectively. Together with the CD3/T cell receptor (TCR) complex and CD4 molecules, MHC-II mediates a critical function in presenting peptides generated from hydrolysis of exogenous antigens by antigen-presenting cells to CD4+ T (helper) cells, thereby either suppressing or inducing an immune response to the peptides. The M5/114.15.2 antibody recognizes a polymorphic determinant shared by the I-Ab, I-Ad, I-Aq, I-Ed, and I-Ek (but not I-Af, I-Ak, or I-As) MHC class II alloantigens expressed on B cells, dendritic cells, macrophages, monocytes, and activated T cells of mice carrying the H-2b, d, p, q, r, and u haplotypes (but not haplotypes H-2f or H-2s). The M5/114 antibody reportedly inhibits I-A-restricted responses by T cells of the H-2b, d, q, and u (but not H-2f, k, or s) haplotypes. It does not react with cells from NOD (H-2g7) mice.