The interleukin-2 receptor (IL-2R) is a heterotrimeric protein expressed on the surface of certain immune cells, such as lymphocytes, that binds and responds to a cytokine called IL-2. IL-2R is consist of three chains, an alpha chain (IL-2Rα, CD25), a beta chain (IL-2Rβ, CD122) and a gamma chain (IL-2Rγ, CD132). The α chain binds IL-2 with low affinity, the combination of β and γ together form a complex that binds IL-2 with intermediate affinity, primarily on memory T cells and NK cells. A model of the IL-2R complex would describe the high affinity receptor as an αβγ trimer, in which all three chains are in contact with the ligand. IL-2 and its receptor have key roles in key functions of the immune system, tolerance and immunity, primarily via their direct effects on T cells. But the function of the soluble IL-2 Rα (sIL-2Rα) is unclear, since it would be expected to be a poor inhibitor of IL-2 because of its low binding affinity. In any case, increased levels of the sIL-2Rα in biological fluids reportedly correlate with increased T and B cell activation and immune system activation.
Monitoring of sIL-2Rα levels in serum provides more detailed insights in several pathological situations such as rheumatoid arthritis, systemic lupus erythematosis (SLE), some leukemias and lymphomas.