T and B lymphocytes apparently recognize antigen in two fundamentally different ways. B cell receptors (antibodies) can bind directly to free antigen, whereas the T cell receptor binds to antigen and cell surface markers on specialized accessory cells, generally designated antigen presenting cells. In humans these cell surface markersof importance for antigen activation of T cells are polymorphic membrance glycoproteins: the Class II HLA-DR molecules {Bergholtz & Thorsby 1977, Hansen et al. 1978, Hirschberg et al. 1979a), and perhaps the MT group of antigens (Thorsby et al. 1982). Antigen therefore most probably must be presented together in complexed form with Class li molecules on the surface of the antigen-presenting cells for T cell activation to occur. Antigen does not necessarily have to be presented together with self MHC molecules, but in certain experimental situations, only with the MHC products" seen" by the T cells at the time of initial sensitization (Thomas & Shevach 1977, Ishii et al. 1981). Additionally, after T cell binding to antigen-MHC complexes in the cell membrane, the antigen-presenting ceil produces a soluble Iymphokine now defined as Interleukin I (Il-I) which is necessary for futher T cell proliferation and differentiation (Blyden & Handschumacher 1977, Smith et al. 1980).

Recentiy we have reported the results of experiments indicating that both human umbilical vein endotheliai cells (EC){Hirschberg et al. 1980, 1981a, 1981b) and epidermal Langerhans cells (LC)(Braathen & Thorsby 1980, Braathen et al. 1980) are capable of cooperating with in vivo sensitized T cells in the immunoproliferative response to various types of antigens. It is the purpose here to review the experimental evidence that EC and LC can replace