IL-10 mediates its biological activities via binding to a heteromeric complex of two distinct type II cytokine receptor subunits, the ligand binding subunit IL-10 R alpha (IL-10 R1) and the signal-transducing accessory subunit IL-10 R beta (IL-10 R2). IL-10 R alpha is specific for IL-10 and expressed by most hemopoietic cells. IL-10 R beta is a widely expressed shared subunit for IL-10, IL-22, IL-26 and the IFNl proteins IL-28A, IL-28B and IL-29.