T cell recognize which antigen in graft rejection -

Ans. is 'a' i.e., MHC II MECHANISM OF TRANSPLANT REJECTION o Rejection is a complex process in which both cell mediated and humoral immunity (antibody mediated) play a role. Cell mediated reaction o It is mediated by T cells o T cell recognizes graft by two pathway. A) Direct Pathway o Ceain cells in the graft express HLA antigen, of these dendritic cells (antigen presenting cells) in the transplant is paicularly impoant. o That means, T cells of the recipient recognize antigen (HLA protein) on the antigen presenting cells of the transplant (donor tissue). o Two types of cells are involved in this pathway. (i) CD-4 T cells --> Recognize HLA-II on antigen presenting cells (dendritic cells) and differentiate into Till cells that secretes cytokines. These cytokines induce inflammatory reaction with local accumulation of mononuclear cells paicularly macrophages which destroye the graft tissue. (ii) CD-8 T cells Recognize HLA-I on APCs and differentiate into cytotoxic-T cells that kill the graft tissue by ---> (i) Perforin - granzyme - dependent killing or (ii) Fas-fas ligand dependent killing. B) Indirect pathway o In this pathway HLA molecule of the graft tissue is taken up by antigen presenting cells of the recepient and is presented to CD4 T cells of the recipient. CD-4 T cells become activated and destroy the graft. o In this pathway cytotoxic CD-8 T cells are not involved. o The direct pathway is the major pathway in acute cellular rejection, whereas the indirect pathway is more impoant in chronic rejection.