Editorials Parasite infections and the risk of asthma and atopy

Common environmental allergens stimulate IgE responses and produce allergic disease, but the allergens that produce the most potent IgE responses in nature originate from helminthic parasites. 1 2 Since parasitic infection is endemic in the majority of the world's population , the relationship between helminthic infection and the IgE response is highly relevant to the understanding of allergic diseases. There is a general consensus that IgE antibody is an important component of the immune resistance to helminthiasis, 5–9 although some conflicting results have been obtained. 3 4 Local IgE reactions can create unfavourable conditions in the gut for intestinal parasites , and IgE can mediate the cytotoxic activity of eosinophils against parasitic larvae. These observations have led to the concept that, from an evolutionary perspective, the primary function of the allergic response may be as part of an anti-parasitic protective mechanism, and allergic disease may be the undesirable reaction towards otherwise inoVensive environmental substances. 10 In developed countries the prevalence of allergic disease has increased over recent years at the same time that improved sanitary conditions have caused the virtual elimination of parasites. 11–13 This increase in allergic diseases has recently been attributed to a Th1/Th2 imbalance caused by diminishing exposure to common bacterial and viral infections, 14 15 but the decrease in helminthic infections should also be considered in this context. Insight into this situation has come from recent immunological studies which have demonstrated that there are two diVerent IgE responses to helminthic infections. The first of these is the host's defensive response to produce IgE specific to parasite antigens. The second response is that the host also exhibits a strong non-specific Th2/interleukin 4 dependent polyclonal synthesis of IgE 16–18 which results in highly elevated total serum IgE levels in parasitised populations. This polyclonal synthesis of IgE may be the helminth's defence mechanism against the eVects of anti-parasite IgE. The polyclonal stimulus can suppress allergic responses by reducing the production of specific IgE antibody, resulting in an inverse relationship between total and specific serum IgE levels. 18–20 The polyclonal IgE also saturates the IgE receptors on mast cells and blocks access to specific IgE, which further inhibits allergic reactions. 12 13 18 20 21 This suppres-sive activity may be the reason for the diminished prevalence of allergic diseases reported in some tropical populations. 22–26 Of great significance is the likelihood that parasites evade the immune response by stimulating excess …