Regulatory T- (T
REG) cells are considered to inhibit the development of both type 1 and type 2 helper T (T
H1 and
TH2) cells. However, the number of T
REG 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, T
REG cells may suppress the onset of allergic disease by downregulating other types of immune cells besides T
H1 and T
H2 cells. The newly discovered interleukin (IL-)17-producing helper T- (T
H17) cells responsible for autoimmune inflammatory diseases may counteract with T
REG cells even in allergic diseases. T
H2 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 T
H1, T
H2, T
H17 and T
REG 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.
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