The M3/84 monoclonal antibody specifically binds to CD107b which is also known as Mac-3, Lysosome-associated membrane protein 2 (LAMP-2/Lamp2/Lamp II), and Lysosomal membrane glycoprotein type B (LGP-B). CD107b is a single-pass type I transmembrane glycoprotein that constitutes a major integral membrane protein of lysosomes and may play a role in lysosomal function. CD107b is also expressed on the surface of mouse mononuclear phagocytes. Surface expression of the 92-110-kDa glycoprotein antigen increases during differentiation of monocytes to activated macrophages and may play a role in adhesion. The M3/84 mAb can detect CD107b antigen on tissue macrophages, thioglycollate-elicited peritoneal macrophages, and some myeloid cell lines, but not on lymphocytes or monocytes. In the bone marrow, very few cells display CD107b antigen on the surface, but a large proportion express cytoplasmic CD107b. The M3/84 antibody has also been reported to stain dendritic cells and endothelium in sections of thymus (both medulla and cortex), lymph nodes, spleen (white pulp), and gut-associated lymphoid tissue.