The Role and Mechanism of Action of Dendritic Cells in Transplantation Immunity

Transplantation immunity provides an important model for studying the function of accessory cells in T-dependent responses. Class II MHC products, when presented as dendritic cells, are powerful immunogens for alloreactive helper T lymphocytes. Class II products on other leukocytes—B cells, macrophages, and epidermal Langerhans cells—are poor immunogens but actively stimulate primed T lymphoblasts. The stimulating activity of dendritic cells can in part be ascribed to their capacity to aggregate T lymphocytes by an antigen-independent mechanism. Dendritic–T cell aggregates represent the milieu in which helper T cell activation occurs in culture.