CD23 is a 45 kD protein, also known as Leu-20, FcεRII, IgE Fc receptor, BLAST-2, B6, and low affinity IgE receptor. It is a member of the Ig family, expressed on most mature B cells, B cells in follicular mantle (but not in proliferating germinal center cells, follicular dendritic cells, monocytes, eosinophils, Langerhans cells, and a subset of T cells (10-15% of tonsillar T cells). CD23 responds to high levels of IgE by downregulating IgE secretion. In human monocytes, CD23 triggering results in release of pro-inflammatory cytokines including TNF-α, IL-1, IL-6, and GM-CSF. CD23 can be proteolytically cleaved to generate soluble CD23 fragments of various molecular weights. Alternate splicing of exon 2 can also generate two cell-surface isoforms of CD23 differing by 6 amino acids in their cytoplasmic region.