Role of regulatory and proinflammatory T-cell populations in allergic diseases.

Regulatory T (Treg) cells are considered to inhibit the development of both type 1 (Th1) and type 2 helper T (Th2) cells. However, it is recently reported that there are reduced numbers of Treg cells in patients with allergic diseases as compared with individuals who have high levels of serum immunoglobulin E and blood eosinophils but are asymptomatic. Therefore, Treg cells may suppress the onset of allergic disease by down-regulating other types of immune cells besides Th1 and Th2 cells. The newly discovered interleukin 17-producing helper T cells that are responsible for autoimmune inflammatory diseases may counteract Treg cells even in allergic diseases. The Th2 cells that are capable of producing of high levels of tumor necrosis factor-α may also be involved in inflammation in allergic diseases. In this review, we further discuss the role of Th1, Th2, interleukin 17-producing helper T cells, and Treg cells in allergic diseases by using the balancing square model and the factors differentiating between patients with clinical manifestations of allergic symptomatic and atopic individuals who are sensitized but asymptomatic.