CD2 is a cell adhesion molecule that is found on the plasma membrane of peripheral T-lymphocytes, thymocytes, natural killer cells, Langerhans cells, and mast cells. It mediates adhesion between T-cells and antigen processing cells, induces co-stimulatory signals in T-cells and T-cell cytokine production, regulates T and NK mediated cytolysis, and inhibits apoptosis of activated peripheral T-cells. CD2 also distinguishes between two subsets of human plasmacytoid dendritic cells with distinct phenotypes and functions. In malignancies, CD2 is expressed on T-cell lymphomas, anaplastic large cell lymphomas, and T-cell leukemias, which helps to differentiate them from negatively staining B-cell neoplasms. The marker is also positive in some mast cell proliferative diseases. References: J Exp Med 1999;190:1383; Immunology 2013;139:48; J Immunol 2009;182:6815