The type I transmembrane glycoprotein Signaling Lymphocytic Activation Molecule (SLAM), also known as CD150, is the prototypic member of the SLAM subgroup of the CD2 protein family. Alternate splicing generates an isoform with a substituted cytoplasmic domain. SLAM is expressed on T cells, B cells, thymocytes, macrophages, dendritic cells, platelets, and hematopoietic stem cells. It is up‑regulated on activated B cells and CD4+ and CD8+ T cells, although it is down‑regulated on Th2 polarized cells. SLAM interacts homophilically with low affinity, and this interaction induces a Th0/Th1 response characterized by clonal expansion, production of IFN-gamma, and increased cytolytic activity of CD8+ T cells. SLAM ligation also promotes B cell activation, allergen-induced eosinophil and mast cell activation, NKT cell development, and the microbicidal response of macrophages to Gram negative bacteria. In humans, SLAM functions as a cellular entry receptor for measles virus.