The 3D12 monoclonal antibody specifically recognizes Human Leukocyte Antigen E (HLA-E) that is widely expressed on leucocytes and some other cell types. Cell surface HLA-E is normally expressed as a noncovalent complex comprised of the ~45 kDa type I transmembrane, HLA-E heavy-chain glycoprotein, the ~12 kDa invariant β2-microglobulin (β2m) light chain, and a short bound peptide. Human HLA-E represents a nonclassical Major Histocompatibility Complex class I (MHC class Ib) molecule that is homologous to mouse H-2 Qa-1. Although structurally related to the classical, highly polymorphic MHC class Ia antigens (HLA-A, -B, -C), HLA-E shows limited polymorphism. HLA-E functions in the regulation or self-nonself discrimination of innate and adaptive immune responses. In addition to binding self peptides, the HLA-E complex can selectively bind and present peptides derived from bacterial or viral pathogen-infected cells, stressed cells, or tumor cells to elicit antigen-specific, HLA-E-restricted CD8+ T cell responses. The cell surface HLA-E complex likewise serves as a ligand for heterodimeric CD94:NKG2A inhibitory and CD94:NKG2C activating receptors that are differentially expressed on NK cells and some T cells. These ligand:receptor interactions can either suppress or promote NK or T cell-mediated responses. The 3D12 antibody reportedly binds to both free or complexed HLA-E heavy chain and can block HLA-E-dependent function.