Description: The B3B4 monoclonal antibody reacts with mouse CD23, a 45 kDa type II transmembrane glycoprotein. CD23 is expressed on resting conventional B cells, and its expression is modulated upon B-cell activation. B-1 cell lineage (CD5+ B cells) does not express CD23. Soluble forms of the antigen have been reported to be biologically active. CD23 is a low affinity receptor for IgE and is thought to play a role in the regulation of IgE response and B-cell activation. CD21 is thought to bind to CD23.