CD23 antigen is a 45-60 kDa membrane glycoprotein that has been identified as a low affinity receptor for IgE production and as a receptor for lymphocyte growth factor. CD23 is found in some mature B-cell lymphomas and in Reed-Sternberg cells in Hodgkin’s disease. Follicular dendritic cells and some activated B cells in germinal centers express CD23 in B cells with high density and mantle zone are weakly stained. The majority of chronic lymphocytic leukemias/small lymphocytic lymphomas are CD23 positive, while mantle cell lymphomas are generally negative, so this marker is useful when applied with other markers to separate the small cell lymphomas. Precursor B and T lymphomas, myeloid neoplasms and mature T-cell lymphomas are CD23 negative and other small cell lymphomas are occasionally positive. CD23 is also positive on activated mature B cells expressing IgM or IgD, monocytes/macrophages, follicular dendritic cells, T cell subsets, eosinophils, Langerhans cells, and small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL).