CD103 is a member of the integrin series of adhesion molecules. This antigen defines a developmentally important subset of T cells, namely mucosal T cells including all IEL (intraepithelial lymphocytes) and ~20% of lamina propria T cells. Expression of CD103 is more restricted outside these mucosal organs, appearing at lower levels on T cell subsets of the lymph node, dendritic epidermis and periphery. In non-epithelial CD103+ T cells there is a bias toward expression on CD8+ cells. This antibody is reported to have signal-inducing activity in a redirected lysis assay and to co-stimulate IEL and CD8+ lymph node cells in conjunction with anti-TCR in vitro, which runs counter to the usual inhibition of CTL by anti-integrins. The property of costimulation is not unique to the mouse since the combination of anti-human alphaIEL plus anti-TCR will produce similar proliferation in human cells, with the exception that CD8+ cells are not stimulated.