IL-15Ra chain is expressed by many cell types including monocytes, dendritic cells, natural killer cells, T cells and fibroblasts. Several isoforms of IL-15Ra exist and are generated either by alternative splicing or by proteolytic cleavage. Hence, IL-15Ra can be found as a membrane receptor or as a soluble receptor (sIL-15Ra); and as most isoforms of the receptor contain a sushi domain allowing very-high-affinity binding of IL-15, signaling or regulatory functions can be attributed to the receptor. Competition between soluble and membrane receptors can result in reduced biological activity of IL-15, though a super agonist effect of the soluble form was also observed. The signaling of IL-15 appears more complicated than for other cytokines because IL-15 primarily exists as a complex bound to IL-15Ra. When IL-15/IL-15Ra complexes are shuttled to the cell surface, they can stimulate opposing cells through the b/cC receptor complex (trans-presentation), or alternatively may allow an autocrine stimulation (cis-presentation).