CD23 is described as a 45 kD protein found on the surface of IgM bearing B cells, eosinophils, macrophages and some T and NK cells. It is also found on EBV-transformed B cells. Additionally, a released form has been described. When first released the CD23 molecule is 35 kD; however, this form is quickly cleaved to obtain the more stable, soluble form which is 25 kD in size. Recently the structure of the CD23 molecule was characterized by cloning and sequencing techniques. Soluble CD23 has been shown to be the B cell growth factor (BCGF). Soluble CD23 is also referred to as Blast-2 and as the low affinity IgE receptor (FCeRII). It has been speculated that CD23 may upregulate IgE synthesis in conjunction with T cell promoted interleukin-4; however, the specific physiologic role of this molecule is not yet well understood. Elevated levels of CD23 have been found in research studies of samples from people with B cell-derived Chronic Lymphocytic Leukemia (B-CLL), with Hyper IgE Syndrome and post-Bone Marrow Transplantation samples. CD23 levels may be proven to relate to disease course in Hairy Cell Leukemia (HCL).