Recognizes a 14kDa protein, identified as S100A9 (also known as Calgranulin B or MRP-14); expressed by granulocytes, monocytes and by tissue macrophages.The protein encoded by this gene is a member of the S100 family of proteins containing 2 EF-hand calcium-binding motifs. S100 proteins are localized in the cytoplasm and/or nucleus of a wide range of cells, and involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. Altered expression of this protein is associated with the disease cystic fibrosis. This MAb reacts with neutrophils, monocytes and macrophages, and has been shown as an important marker for identifying macrophages in tissue sections. Among cells that are now recognized as macrophages are histiocytes, Kupffer cells, osteoclasts, microglial cells, synovial type A cells, interdigitating cells, and Langerhans cells (in normal tissues) and epithelioid cells and Langerhans-type and foreign-body-type multinucleated giant cells (in inflamed tissues).