CD43 (Leukosialin, sialophorin, SPN) is a transmembrane protein expressed on the surface of T and B lymphocytes and granulocytes. It is the major sialic acid rich protein found on the plasma membrane of lymphocytes, and functions in the immune system as a co-receptor for various lymphoid cells and in the activation of T-cells. Defective CD43 is associated with Wiskott-Aldrich syndrome, described by an inherited shortage of platelets and insufficient T-lymphocyte function. In immunohistochemistry, CD43 is positive in normal T cells, plasma cells, and macrophages. In cancers, it is positive in low-grade B cell lymphomas and peripheral T-cell lymphomas. References: Immunodefic Rev. 1990;2(2):151-74, PMID: 2223062; Immunol Res. 1999;20(2):89-99, PMID: 10580634