STAT3 and Alzheimer disease: Indeed, IL-31 (300–1000 ng/mL; 24 h), a highly “AD-relevant” cytokine mostly produced by several immune cells [252], increased β-endorphin release from primary human epidermal keratinocytes via activating signal transducer and activator of transcription (STAT)-3, and a significant correlation in the blood levels of IL-31 and β-endorphin was also demonstrated in AD patients [142].