**3. Immunological basis of allograft rejection**

With regard to liver transplantation, grafts mainly originate from different members of the human species. The genetically encoded immunologically mediated barrier to transplantation was recognised and defined over the course of the last century. The immunological study of transplantation has played a pivotal role in the development of clinical transplantation. Although the first successful liver transplant was between identical twins, the development of transplantation as an important facet of modern medical therapy required the introduction of immunosuppression so as to prevent and treat the rejection of allografts (Liu L U et al., 2002; Yoshizawa A et al., 2006; Braillon A, 2009). The process of rejection is very complicated and has been shown to be caused by transplantation antigens, including major histocompatibility antigens, minor histocompatibility antigens and other alloantigens. In addition, infiltrating leucocytes also launch the process, and it exhibits specificity and memory and is prevented by lymphocyte depletion (Gowans J L, 1962). The major histocompatibility complex (MHC) was identified as encoding the dominant transplantation antigens, and these were shown to be identical to serologically defined human leucocyte antigens (HLA), and subsequently to the elements responsible for the selfrestriction of immunological responses to conventional antigens. The molecular and cellular basis of graft rejection will be described in the next section (Figure 2).

Fig. 2. The evolution of the immune response after liver transplantation. MHC, major histocompatibility complex; TCR, T cell receptor; APC, antigen presenting cell; IFN, interferon; TNF, tumour necrosis factor
