What do dry skin, diet, dogs, dribble and Vitamin D all have in common? This is a question that has been deliberated by immunologists for many years now. The so-called ‘5 Ds’ have all been posited to play a part in the formation of allergies. Dry skin and diet fall under the ‘dual-allergen exposure hypothesis’, dogs and dribble the ‘hygiene hypothesis’ and Vitamin D, yes, you guessed it, the ‘Vitamin D hypothesis’. An allergy is essentially that scenario at the family Christmas lunch where your antiquated uncle makes an unwanted comment to cause a hugely over exaggerated response from your cousin, who just ‘found themselves’ in Thailand. In scientific terms, that scenario is labelled ‘an inappropriate immune response to otherwise harmless antigens’. An antigen is any foreign molecule that elicits an immune response, whilst an immune response can range from disturbances to the skin (hives, for example) to life-threatening hypovolemic shock and respiratory compromise. The real question to ask, then, is why do certain antigens cause allergies in some people but not in others? The ‘dual-allergen exposure hypothesis’ suggests that allergen sensitisation occurs in children with dry skin, often in the form of childhood eczema. It is thought that a disrupted skin barrier allows environmental exposure to the allergen via the skin to result resulting in allergen sensitisation, the process of antibody production behind the allergic response we see. This hypothesis thus explains the high association of children with eczema that go on to develop food allergies later on in their life. However, this school of thought is applicable only to a small group of children with infant eczema. When thinking in broader terms, the ‘hygiene hypothesis’ has been a popular notion used to elucidate the development of allergies. This idea posits that the environment in which children grow up in goes a long way to determine if they will develop allergies later on in life. Numerous different factors fall under this umbrella hypothesis: vaginal delivery versus caesarian delivery, rural farm live versus urban life, pet dogs, childcare attendance and presence of older siblings to name a few. Fundamentally, this hypothesis addresses the different types of microorganisms you would have been in contact with as a child. The larger this number is, the lower your chances of developing an allergy. In order to develop tolerance to antigens as a child, it is important to be exposed to diverse microorganisms.
The last major hypothesis is the ‘Vitamin D hypothesis’, an interesting concept that has grown from numerous observations. Firstly, there are higher rates of food allergy and hospital admissions for food-related anaphylaxis in children that live further from the equator than those that live nearer the equator. Secondly, children born in autumn or winter are more likely to develop allergies compared to those who were born in spring or summer. Following these observations, studies have shown that vitamin D has an important role in regulating several different immune functions. Rather unsatisfyingly, it seems that allergies develop due to a number of different factors. It is hoped that in the future through further genetic testing and observational studies that there will be a clearer idea of what causes allergies.