The monoclonal antibody MU14-2A5 recognizes S100A9. The calcium-binding, migration inhibitory factor-related proteins, MRP-8 (S100A8) and MRP-14 (S100A9) belong to the S100 protein family. MRP14 is co-expressed with human MRP8, a homologue protein in myeloid cells, and plays an essential role in calcium dependent functions during inflammation. This role includes the activation of Mac-1, the β2 integrin which is involved in neutrophil adhesion to endothelial cells. MRP14 is distinguished from other S100 member proteins by its long C-terminal region.The expression of MRP-8 (S100A8) and MRP-14 (S100A9) is largely confined to the cytosol of neutrophils and monocytes. The complex formation of these proteins is a calcium-dependent process. The S100A8/A9 heterocomplex, also called MRP-8/MRP-14 complex or calprotectin, comprises 60% of the cytoplasmic protein fraction of circulating polymorphonuclear granulocytes and is also found in monocytes, macrophages and ileal tissue eosinophils. In inflammatory conditions small venueles stain with both anti-S100A8 and S100A9. The staining of the two subunits is always coincident. The S100A8/A9 complex has antibacterial, antifungal and immunomodulating and antiproliferative effects. Besides this it is a potent chemotactic factor for neutrophils.