CD23 (FCER2) is a C-type lectin found on the surface of B-cells that acts as a receptor for Immunoglobulin E (IgE). CD23 can be released from the plasma membrane to act as a soluble IgE-binding protein with cytokine functionality. Free CD23 proteins may be cleaved into a range of isoforms of different molecular weights. Functionally, CD23 plays a regulatory role in cell adhesion processes and receptor-mediated feedback loops of the immune response. In immunohistochemistry, CD23 is a membranous marker for activated B-cells, macrophages, some follicular dendritic cells in germinal centers and a subset of B cells in the mantle zone of lymphoid tissues. It is positive in small lymphocytic lymphomas and B-cell chronic lymphocytic leukemia, but not mantle cell lymphomas. References: Clin Exp Immunol. 2010 Oct; 162(1): 12–23, PMID: 20831712; Eur J Immunol. 1993 Sep;23(9):2066-71, PMID: 8370388