CD23 (FCE2) is a type II integral membrane glycoprotein that is expressed on mature B cells, monocytes, eosinophils, platelets and dendritic cells. CD23 associates as an oligomer where cooperative binding of at least two lectin domains is required for high affinity IgE binding to CD23. It may play a role in antigen presentation by B cells by interacting with CD40. CD23 has been shown to be associated with the Fyn tyrosine kinase. The truncated molecule can be secreted, then function as a potent mitogenic growth factor. CD23 is expressed on a subpopulation of peripheral blood cells, B-lymphocytes and on EBV transformed B lymphoblastoid cell lines. CD23 is also detected in neoplastic cells from cases of B cell chronic lymphocytic leukemia.