This lecture covers the history, mechanisms, and clinical applications of transplant rejection, focusing on the cellular immune response against alloantigens. It explains the direct and indirect recognition of allogeneic molecules by T cells, the phases of sensitization, and the effector mechanisms involved in graft destruction. Additionally, it discusses hyperacute, acute, and chronic rejection, as well as immunosuppressive therapies. The instructor details the manifestations of transplant rejection, including endothelial inflammation, smooth vascular proliferation, and chronic inflammation, which can lead to a gradual decline in graft function.