Anti-CD9 recognizes a human 24-kiloDalton (kDa) single-chain cell-surface glycoprotein (p24) belonging to the tetraspanin family which transverse the membrane four times. The CD9 antigen has a very broad tissue distribution. It is present on basophils，eosinophils，monocytes，pre-B cells，B cells，and various leukemic cell lines (erythroid，myeloid，some T-lymphoid，pre–B-lymphoid). It is also found on follicular center cells，sinus histocytes，macrophages，Kupffer cells，osteoclasts，hepatocytes，bile duct endothelium，renal glomeruli，proximal and distal tubuli，epithelia (intercellular spinous spaces) of skin and mucosa，fibroblasts，connective tissue，endothelium，smooth muscle，cardiac muscle，synovial lining cells，brain white matter，peripheral nerves and human keratin. In addition to being a co-stimulatory molecule，CD9 antigen activates platelets and induces mitogenesis and kinase activity and is involved in cell migration，cell adhesion，and integrin signaling.
