IL17A and psoriasis: However, the ligands for the heterodimeric receptor complex of IL-17RA and IL-17RD are not clear, though a study about psoriasis-like skin inflammation showed that IL-17RA/RD directly bound to IL-17A but not IL-17F or IL-17A/F heterodimer to mediate the proinflammatory gene expression downstream of IL-17A (Su et al., 2019).