Cytokine receptors frequently form dimers upon ligand binding, and these can be heterodimers or homodimers. Explain how this ability of the receptors to form different combinations of dimers allows three unique cytokines, each with a unique action, to arise from only two cytokine receptors.

The interaction of cell surface receptor extracellular domains with secreted ligands is essential to most types of cell signaling and cell-cell communication. This initial step and the subsequent activation of membrane-proximal and - distal intracellular signaling cascades lead to specific, although often redundant, cellular responses that control cell proliferation, differentiation, maturation, and survival. Cell surface receptors usually bind their ligands through highly specific molecular interactions to provide the tight regulation necessary for control of physiological responses. However, researchers increasingly appreciate that many receptor systems exhibit, to a greater or lesser extent, cross-reactivity with a spectrum of different ligands