In plasma of both PwRRMS and people with SPMS (PwSPMS), PGD2 levels are found to be elevated [20], where it is proposed to have both anti- and pro-inflammatory properties depending on the G protein-coupled receptor (GPCR) it interacts with: the D prostanoid receptor (DP1) or the chemoattractant receptor–homologous molecule on Th2 cells (CRTH2, also known as DP2) (Table 1). Here, PTGDR2 is linked to secondary progressive multiple sclerosis.