**1. Introduction**

Transplantation is the most beneficial approach to treat diseases, manifested by irreversible changes of the liver parenchyma. The success of transplantation depends on both the surgical operation and the development of an immune tolerant milieu in the post-transplantation period. In solid organ transplantation, the immunological mechanisms that are naturally dedicated to the defense from foreign antigens (microbial, viral etc.), are directed towards HLA (MHC) molecules and allo-antigens of the graft. This powerful immune reaction may destroy the graft and compromise the beneficial effect of the transplant. In the routine clinical practice, the control of effector immune function is achieved through immune suppressive therapy. However, in kidney and liver transplantation a spontaneous development of immune tolerance where a particular T cells subset – regulatory T cells (Tregs) is supposed to play important role [1].

Thus, recent achievements in transplantation research motivate the focus on the immunological mechanisms in two directions. From one side, this is the continuous investigation on new and more relevant biomarkers for the monitoring of the posttransplantation period and prediction of graft rejection. From the other side, is the need of new therapeutic opportunities that might be influenced by the scientific research on the fine immune mechanisms [2].

#### **Figure 1.**

*Principal medical conditions that require liver transplantation.*

Studies on Tregs are particularly intense in the field of transplantology precisely in connection with their suppressive function. Lots of data in the literature on their behavior in transplantation of solid organs as well as stem cells is present. While in kidney, heart and other transplants already have outlined trends in the dynamics and even the therapeutic application of Tregs [3], the situation with liver transplantation (LT) is more special.

Patients with life-limiting liver disease, which may present in the form of acute liver failure, end-stage chronic liver disease, hepatic malignancy, or inborn metabolic disorders need LT - liver function is heavily impaired as a result of irreversible morphological changes (**Figure 1**).

Whatever the cause, the outcome of liver transplantation depends on three main factors: the clinical approach, the immune characteristics of the liver, and the therapeutic provision of immunological tolerance.
