IGHE and Alzheimer disease: On the other hand, AD can be divided into extrinsic and intrinsic groups depending on the presence or lack of immunoglobulin (Ig) E. This Ig cannot be considered as a good biomarker either because some patients have shown to develop high IgE levels throughout the course of the disease [18] and elevated allergen-specific IgE levels have also been found in healthy individuals [19].