The HI9a antibody reacts with an extracellular epitope on human CD9, a 24 kDa single-chain type III transmembrane glycoprotein. CD9 is broadly expressed, including on the surface of monocytes, developing B cells, activated T cells, granulocytes, epithelial cells, endothelial cells, and platelets (mainly the α-granules) and is also a marker characteristic of extracellular vesicles.

CD9 belongs to the tetraspanin family of proteins, which contain four transmembrane domains, two extracellular loops, and short cytoplasmic N- and C-termini. It is involved in mediating several cellular processes, including angiogenesis, by promoting cell adhesion, migration, and signal transduction, and is involved in cell fusion processes linked to fertilization, osteoclastogenesis, and myogenesis.