Regulatory T- (TREG) cells are considered to inhibit the development of both type 1 and type 2 helper T (TH1 and TH2) cells. However, the number of TREG cells in patients with allergic diseases who have high levels of serum IgE and blood eosinophils is reduced as compared to individuals who have similarly high levels of IgE and eosinophils but are asymptomatic. Therefore, TREG cells may suppress the onset of allergic disease by downregulating other types of immune cells besides TH1 and TH2 cells. The newly discovered interleukin (IL-)17-producing helper T- (TH17) cells responsible for autoimmune inflammatory diseases may counteract with TREG cells even in allergic diseases. TH2 cells capable of producing of high levels of tumor necrosis factor (TNF)-a may also be involved in the inflammation in allergic diseases. In this review, the role of TH1, TH2, TH17 and TREG cells in allergic diseases is further discussed by using the balancing square model and the factors differentiating between patients with clinical manifestations of allergic symptomatic versus atopic individuals who are sensitized but asymptomatic.
2009 The Japanese Society of Toxicology