Interleukin-2 is an important immune system regulator necessary for the clone expansion and survival of activated lymphocytes T. Its effects are mediated by the trimer cell surface receptor IL-2a, consisting of the α, β, and γ chains. The IL-2a (CD25, T-cell activation antigen, TAC) is expressed only by the already-activated T lymphocytes. Therefore, it is of special significance to the selective immunosuppressive treatment, and research has been focused on the development of effective and safe anti-IL-2 antibodies. By the use of recombinant gene technology, the mouse anti-Tac antibodies have been modified, leading to the presentation of two chimeric mouse/human anti-Tac antibodies in the year 1998: basiliximab (Simulect) and daclizumab (Zenapax). These drugs act by binding the IL-2a receptor's α chain, preventing the IL-2 induced clonal expansion of activated lymphocytes and shortening their survival. They are used in the prophylaxis of the acute organ rejection after bilateral kidney transplantation, both being similarly effective and with only few side-effects.