Immunology - Allergy, parasites, and the hygiene hypothesis

The increase of allergic diseases in the industrialized world has often been explained by a decline in infections during childhood. The immunological explanation has been put into the context of the functional T cell subsets known as T helper 1 (T(H)1) and T helper 2 (T(H)2) that display polarized cytokine profiles. It has been argued that bacterial, and viral infections during early life direct the maturing immune system toward T(H)1, which counterbalance proallergic responses of T(H)2 cells. Thus, a reduction in the overall microbial burden will result in weak T(H)1 imprinting and unrestrained T(H)2 responses that allow an increase in allergy. This notion is contradicted by observations that the prevalence of T(H)1-autoimmune diseases is also increasing and that T(H)2-skewed parasitic worm (helminth) infections are not associated with allergy. More recently, elevations of anti-inflammatory cytokines, such as interieukin-10, that occur during tong-term helminth infections have been shown to be inversely correlated with allergy. The induction of a robust anti-inflammatory regulatory network by persistent immune challenge offers a unifying explanation for the observed inverse association of many infections with allergic disorders.