The IL-2 receptor is a complex of three distinct polypeptide chains: (i) the α chain which binds IL-2 with low affinity, (ii) the β chain that binds IL-2 with high affinity, and (ii) the common γ chain ( γ C ) that does not bind IL-2. The high affinity receptor complex is an α / β / γ heterotrimer with a K d of 1.3 x 10 -11 M. In mouse spleen, CD122 is expresed on approx. 30 % of CD8 + cells and all NK cells, but < 1 % of B cells and CD4 + T lymphocytes. In the thymus, its expression is confined to CD4 - CD8 + single positive and CD4 - CD8 - double negative cells. Cytoplasmic regions of the IL-2R β chain are involved in IL-2-mediated cellular signaling and, via the interaction of IL-2 and its receptor complex, may be involved in the generation and differentiation of T lymphocytes.