IGHE and Alzheimer disease: In general, AD is roughly divided into two distinct variants depending on the presence or absence of involvement in the pathology of IgE allergy: (1) extrinsic IgE-allergic AD, which occurs in the context of sensitization to environmental allergens that produce allergen-specific IgEs and is accompanied by elevated serum total IgE levels (more than approximately 150 or 400 IU/mL, according to standards of the individual facility), and (2) intrinsic non–IgE-allergic AD, with no detectable sensitization to environmental allergens and with normal serum total IgE levels [113].