The recognition of a tissue allograft as foreign and the induction of an immune response against that graft is poorly understood, but is obviously extremely complex and is influenced by a number of factors. MHC: Incompatibility for the MHC of the donor is obviously important, and incompatibility for both Class I (eg. HLA-A, B) or Class II (eg. HLA-DR) antigens will induce the response. However in some circumstances presentation of Class I antigen only on viable cells will induce suppression rather than a response leading to rejection. Presentation of histocompatibility antigens: Two theoretical methods exist by which the host may recognise foreign alloantigen. In the first host antigen-presenting cells present alloantigen in association with host Ia to host TH (the conventional form of antigen presentation), while in the second passenger leucocytes (dendritic cells) in the donor allograft present incompatible Ia (Class II) antigen directly to host TH cells, or indeed if Ia is identical with the host they may present Class I antigen to host TH cells. There is considerable experimental evidence which suggests an important role for the passenger leucocyte in the induction of the allograft response. Minor histocompatibility antigens: Not a great deal is known about these systems in man, but there are no doubt very many. From our knowledge of the behaviour of skin and cardiac allografts in the mouse transplanted across major and minor barriers, minor antigens may induce an immune response to an allograft especially in the presence of compatibility for the MHC.(ABSTRACT TRUNCATED AT 250 WORDS)
