Coincidence of immune-mediated diseases driven by Th1 and Th2 subsets suggests a common aetiology. A population-based study using computerized General Practice data

The recent rise in the prevalence of immune-mediated diseases has been attributed to environmental factors such as a lack of microbial challenge, or dietary change, that deviate the overall balance between mutually antagonistic subsets of T helper (Th) cells. An alternative proposal is that recent environmental changes have resulted in an immune system that is more likely to produce both Th1 and Th2 responses against benign antigens. The prediction of this hypothesis, that Th1 and Th2-mediated diseases are not mutually exclusive, and may be positively associated, is tested here in a whole population. Data from General Practices participating in the Scottish Continuous Morbidity Recording (CMR) project were used to determine the coincidence of the major Th2-mediated atopic diseases; asthma, eczema and allergic rhinitis, with the Th1-mediated autoimmune conditions; type I diabetes, rheumatoid arthritis and psoriasis. We also identified the prescription rates of inhaled therapy for asthma in patients with Th1-mediated disease. There was a significant increase in the risk of presenting with a Th1-mediated autoimmune condition in patients with a history of allergic disease (standardized prevalence ratio (95% confidence interval) 1.28 (1.18-1.37)). Likewise, the standardized prevalence ratios of presenting with either eczema (1.67 (1.48-1.87)) or allergic rhinitis (1.22 (1.02-1.44)) were significantly increased in subjects with a history of Th1-mediated disease. There was a particularly strong association between current psoriasis and current eczema (standardized prevalence ratio ofpsoriasis in subjects with eczema 2.88, 95% confidence interval (CI) 2.38-3.45). There was also a significant increase in prescriptions for inhaled asthma therapy in patients with Th1 disease. It is concluded that Th1- and Th2-mediated diseases are significantly associated in a large General Practice population. This finding supports the proposal that autoimmune and atopic diseases share risk factors that increase the propensity of the immune system to generate both Th1- and Th2-mediated inappropriate responses to non-pathological antigens.