IL21 and allergic disease: Within the IL-2 subfamily there are two CD4+ T cell associated cytokines; IL-2 is a key inducer of Treg cells, which modulate immune responses, promote feto-maternal tolerance, and help avoid allergy and autoimmunity [17], while IL-21 is the effector cytokine of TFH cells, which contribute to antibody affinity maturation and immune memory [8].