Allergy and Eczema: a New Explanation

The rising incidence of allergy and allergic diseases in the developed world might be attributable to excessive washing with harsh soaps and abrasive skin care products which strip away a protective layer of skin.

Researchers at the UCL Institute of Child Health/Great Ormond Street Hospital propose a new mechanism to explain the high incidence of atopic dermatitis [eczema] and other allergic diseases in the developed world. They offer this as an alternative to the so-called ‘hygiene hypothesis’ which claims that reduced exposure to infectious disease, particularly in childhood, weakens the immune system and allows allergic responses to emerge. Robin Callard and John Harper’s paper published on-line in Trends in Immunology brings together two separate lines of work; genetic investigation into defects of the protective skin layer, and direct experiment.Professor Callard explains: “Our hypothesis, like the ‘hygiene hypothesis’, assumes that the modern obsession with cleanliness drives the higher levels of allergic disease. But the mechanism is radically different. Too much washing with strong soaps, using exfoliants and other such skin care products, and perhaps biological washing powders could be stripping away the skin’s outer protective layer resulting in allergic responses to allergens in susceptible individuals. “We have shown that children and adults with a rare genetic skin disease who develop atopic dermatitis and allergy also have a weakened skin protective layer. In the lab, we have shown that if the outer protective layer of the skin is stripped away using something as simple as sellotape, allergens and other proteins are able to penetrate the skin and be taken up by specialised cells called Langerhans cells in the epidermis. The Langerhans cells then move from the skin to the local lymph nodes and induce the classic Th2 allergic immune response.”This is a radically different mechanism to the ‘hygiene hypothesis’, which is based on the idea that infections in childhood bias the immune system towards a Th1 response and away from the allergic Th2 immune response. Reduced exposure to infection in the developed world is therefore supposed to enable unwanted Th2 allergic responses. Despite its popularity over the past 20 years there is very little supporting evidence for the ‘hygiene hypothesis’. In contrast, there is mounting evidence from both studies of rare genetic conditions and our lab work to support an important role for abnormalities in the outer protective layer of the skin in allowing allergic sensitization. We think this proposal deserves serious consideration and discussion.”Professor John Harper said, “I think from a clinical viewpoint, it is important to stress we think it is over zealous washing using harsh detergents and/or abrasive skin care products that can damage the skin that are likely to be at fault and not normal washing or normal standards of cleanliness. For example, we actively encourage our dermatitis patients to bathe frequently: that advice does not change. Good standards of hygiene are clearly important to prevent spreading of unpleasant diseases. But if this proposed mechanism does turn out to be true, we may be able to reduce the incidence of these diseases by developing new treatments which specifically target the outer protective layer of the skin.” (Source: UCL Institute of Child Health : June 2007)