This MAb stains the cytoplasm of macrophages and histiocytes in hematopoietic organs, Kupffer’s cells of the liver and Langerhan’s cells of the skin. It also stains the mantle zone B-lymphocytes of the lymph node and spleen, spermatogonia, and chief cells of the stomach. S100A9 is expressed by macrophages in acutely inflamed tissues and in chronic inflammation. It is detected in peripheral blood leukocytes, in neutrophils and granulocytes. It is present at sites of vascular inflammation. S100A9 is also expressed in epithelial cells constitutively or induced during dermatoses. S100A9 is a Calcium-binding protein. It has antimicrobial activity towards bacteria and fungi. It is important for resistance to invasion by pathogenic bacteria. It up-regulates transcription of genes that are under the control of NF-kappa-B. S100A9 plays a role in the development of endotoxic shock in response to bacterial lipopolysaccharide (LPS). It promotes tubulin polymerization when unphosphorylated. It also promotes phagocyte migration and infiltration of granulocytes at sites of wounding. It plays a role as a pro-inflammatory mediator in acute and chronic inflammation and up-regulates the release of IL8 and cell-surface expression of ICAM1.