The EBVCS-5 antibody is specific for human CD23, also known as the low affinity IgE receptor (FceRII), a 45 kDa type II transmembrane protein and member of the immunoglobulin superfamily. It is expressed on most B cells and is upregulated upon activation. It is also present on mantle zone B cells, eosinophils, monocytes and a subset of T cells and platelets. CD23 plays a role in B cell development and differentiation, and also functions to regulate IgE production. Soluble secreted forms of CD23 have been reported to bind IgE, stimulate release of pro-inflammatory cytokines from monocytes, and play a role in B cell differentiation.