The alloresponse.

The alloresponse can be divided into two components. The first of these is allorecognition, which refers to the recognition of antigens, expressed on the surface of cells of non-self origin, by the host's lymphocytes. The second part is the immune effector mechanisms generated by this recognition process. The molecules recognised have been termed histocompatibility antigens and fall into two categories. The strongest responses are provoked by allogeneic major histocompatibility complex (MHC) antigens. T cells recognise these antigens either directly or after being processed like conventional antigens by antigen-presenting cells, in what has been termed indirect presentation. In the context of MHC identity, responses are observed against the second category of antigens, namely minor histocompatibility antigens (mHAgs). Although weaker, these responses are of clinical importance, particularly in bone marrow transplant recipients. CD4+ T cells play a central role in orchestrating the immune response to alloantigens. They secrete cytokines to attract effector cells, such as macrophages and CD8+ T cells, into the graft and are able to interact with B cells that will secrete highly specific alloreactive antibodies. In clinical terms, the result of the immune response to transplanted allografts can be classified as hyperacute rejection, acute and chronic rejection. The immunological effector mechanisms involved in each of these processes are discussed.