PTGDR2 and Alzheimer disease: PGD2 plays a multifaceted role in the pathophysiology of AD, contributing to inflammation, pruritus, immune dysregulation, and skin barrier dysfunction.39 PGD2 is the main PG produced by mast cells in AD and acts on 2 different receptors: D-prostanoid receptor 1 (DP1) and chemoattractant receptor-homologous molecule expressed on TH2 cells (CRTH2).