IL31RA and Alzheimer disease: Since the initial identification of the T cell-derived cytokine IL-31 in 2004 [59], AD patients were found to have elevated expression levels of IL-31 in skin-infiltrating cells (e.g., mononuclear cells) and IL-31 receptor subunit α (IL-31Rα) in keratinocytes and nerve fibers in the dermis [60].