MRGPRX2 and anaphylaxis: Their quantification by flow cytometry is a reliable marker of mast cell activation, which is often used for the diagnosis of allergies and anaphylaxis in mast cell activation tests.49,50 Thus, the expression of CD107a and CD63 on the cell surface was determined to confirm the effects of PSB-172656 on SP-induced MRGPRX2-mediated degranulation.