IL-2Rβ, also known as IL-15RB, is a type I transmembrane protein. IL-2Rβ is one subunit of IL-2 receptor, which is constituted by α (CD25), β (CD122), and common γ (CD132) chains. The IL-2Rβ associates with either the common γ subunit alone or with the γc subunit and the IL-2Rα subunit to generate intermediate or high affinity IL-2 receptor complexes, respectively. IL-2Rβ and γc are also shared by the IL-15 receptor. Upon IL-2 ligation, the IL-2 receptor complexes induce the downstream signaling pathways, including JAK/STAT, PI3K and Ras/MAPK pathways. Though the IL-2 receptor has no intrinsic kinase activity, IL-2Rβ and γc were found to be associated with tyrosine kinases, JAK1 and JAK3, respectively, and recruit adaptor proteins. Phosphorylated IL-2Rβ is necessary for transducing the signaling. IL-2Rβ is constitutively expressed by NK cells and at lower levels by T cells, B cells, monocytes, and macrophages, but its expression levels can be upregulated by activation. It has been reported that IL-2Rβ is cleaved and released from the cell surface though the function of soluble form in vivo is not clear yet. High level of soluble IL-2R in serum has been observed in autoimmune diseases like systemic lupus erythematosus and rheumatoid arthritis. In IL-2Rβ gene deficient mice, T cells are activated and it is observed exhaustive B cell differentiation, indicating that IL-2Rβ is involved in the maintenance of T cell homeostasis. IL-2Rβ is associated with autoimmune diseases. It has been reported that IL-2Rβ expression is reduced on CD4+ T cell in ANCA (antineutrophil-cytoplasmic-antibodies)-associated vasculitis. In addition, polymorphisms in IL-2Rβ gene were identified to associated with inflammatory bowel disease based on genome-wide studies.