S1PR1 and lymphopenia: Compounds targeting this receptor have been primarily developed as receptor‐desensitizing agents with associated peripheral blood lymphopenia, and this has been exploited in the approval of 3 drugs for multiple sclerosis, fingolimod (a nonselective S1P1/3/4/5 agonist), and more recently siponimod and ozanimod (S1P1/5 agonists) [3, 4, 5].