CD4 and allergic disease: Activated CD4+ T-cells can differentiate into T helper 1 (Th1) CD4+ cells, which are primarily involved in cell-mediated antigen responses, T helper 2 (Th2) CD4+ cells, involved in B-cell responses to antigens and allergy, T helper 17 (Th17) CD4+ cells, which are induced by cytokines to enhance the inflammatory response, T-regulatory (Treg) CD4+ cells, which suppress immune response, and other subtypes [31].