The UCHT2 antibody reacts with CD5, an ~67 kDa single-chain type I glycoprotein and member of the scavenger receptor superfamily, which is constitutively expressed on thymocytes, T cells, B cell subsets, peripheral blood dendritic cells and some leukemia and lymphoma cells, including chronic B lymphocytic leukemia (B-CLL) cells. CD5 is expressed at low levels on thymocytes and at high density on mature T cells. Putative ligands include CD5L and CD72. CD5 modulates T and B cell receptor signaling, thymocyte maturation, and T cell/B cell interactions via its physical and functional associations with the T cell receptor/CD3 complex and the B cell receptor. Studies with CD5 knockout mice indicate that CD5 negatively regulates antigen receptor-mediated signaling in thymocytes and mature T cells. The UCHT2 antibody binds to epitope 2 located in the extracellular domain of CD5.