The HB15e antibody reacts with CD83, an ~45 kDa type I transmembrane glycoprotein and member of the Siglec protein family, containing an extracellular V-type Ig-like domain and a C-terminal cytoplasmic tail. A soluble form of CD83 has also been identified. CD83 is expressed predominantly on circulating and tissue dendritic cells, including Langerhan cells and interdigitating reticulum cells. It is expressed weakly on activated lymphocytes and may also be found on neutrophils, monocytes and macrophages. Expression of CD83 is typically transient, except on dendritic cells, and correlates with upregulation of HLA class II antigen. Its function, though ill-defined, is essential for the development and maintenance of the immune system and evidence suggests it may be involved in cell adhesion and signaling, antigen presentation, and lymphocyte activation. The soluble form of CD83 appears to be immunosuppressive. The epitope on CD83 bound by the HB15e antibody reportedly overlaps with that of antibody 1E11.