The principal metabolites associated with AD are tryptophan, indoleacetic acid (increased in patients with AD and elevated total serum IgE), glycolic acid, taurocholic acid, taurochenodeoxycholic and glycochenodeoxycholic (decreased in patients with AD independent of the total serum IgE level), and cholic and chenodeoxycholic (increased in patients with AD and elevated total serum IgE) [172,173,174]. Here, IGHE is linked to Alzheimer disease.