KLK4 and Alzheimer disease: Exposure to several exogenous proteolytic enzymes (derived from bacteria or house dust mites) or from endogenous sources (tryptase, trypsin, chymase, kallikrein, especially KLK5) is observed in the skin of patients with AD, which are released by keratinocytes or by immune cells in the inflammatory process, highlighting the relevance of this pathway in the early stages of the disease.