Schematic representation of intracellular signaling in graft rejection

Once T cell activation occurs, a chain of intracellular events is triggered under the influence of a variety of growth and differentiation factors, of which the best characterized is IL-2. These events lead to clonal expansion of antigen-specific T cells. Such cells are differentiated into CD8+ cytotoxic T cells (which directly induce donor cell death), and CD4+ helper T cells (which help B cells produce alloantibodies and help macrophages induce delayed type hypersensitivity responses). All of these mechanisms are active in graft rejection.