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Background: Infection with Mycobacterium tuberculosis induces a type-1 immune response, whereas intestinal parasites elicit a type-2 response. Given that type-1 and type-2 responses inhibit each other, we investigated if M tuberculosis downregulates serum IgE, a marker of a type-2 response.Methods: A prospective study was done in the Western Cape Province of South Africa, where tuberculosis and intestinal-parasite infection are common. Total serum IgE was determined for 37 controls and for 33 adolescent patients at presentation with tuberculosis and after successful completion of treatment. IgE specific for ascaris and allergens were measured in a subset of these individuals. Mantoux skin tests were done on 35 controls and on 31 patients at diagnosis.Findings: Total IgE concentrations were high in controls (mean 313 kU/L) and in patients before treatment (mean 457 kU/L, p=0.085) and declined in all patients following successful treatment (mean 175 kU/L, p<0.0001). Posttreatment IgE concentrations did not differ from concentrations in controls. Ascaris-specific IgE was lower in controls (mean 1.73 kU/L) than in patients before treatment (4.62 kU/L, p=0.023) and was 2.39 kU/L in patients after treatment (p=0.0625). Tuberculin induration correlated inversely with IgE in patients but not in controls.Interpretation: Infection with M tuberculosis as such is not incompatible with a prominent IgE response. IgE concentrations decreased after successful treatment of tuberculosis, showing that IgE concentrations in human beings can be downregulated under these circumstances, presumably due to enhancement of a type-1 response. [ABSTRACT FROM AUTHOR]