CD4 and allergic asthma: The support of this phenomenon in patients is evidence that asthmatics have Th2 cells in their lungs (7, 8), healthy lungs have memory CD4+ and CD8+ T cells expressing TRM markers, e.g., CD69 and CD103 (18–21), patients with HLA–mismatched lung transplants have donor TRMs for more than 1 year after transplant (22), and children with allergic asthma improve when allergen is removed from their environment, only to relapse upon re-exposure to allergen (23).