So far, only monoclonal antibodies directed at functional target molecules on lymphocyte surface membrane have been proven useful in preventing or reversing allograft rejection episodes. Antibodies directed against light chain (P55) of interleukin 2 (IL2) receptor and able to interfere with IL2 binding on the IL2-receptor in its high-affinity conformation (only expressed on activated T cells) are effective in various animal models and recently in a preventive protocol in human kidney graft recipients. Thus, IL2-receptor targeting emerges as a new therapeutic strategy involving only a small pool of progenitors committed against donor antigens. Furthermore, membranous or soluble forms of P55, as indicators of the presence of alloreactive clones in the graft or at the peripheral level, may offer new tools for monitoring the rejection process.