The Root of Allergies

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 positsthat the environment in which children grow up ingoes a long way to determine if they will developallergies later on in life. Numerous different factorsfall under this umbrella hypothesis: vaginal deliveryversus caesarian delivery, rural farm live versus urbanlife, pet dogs, childcare attendance and presence ofolder siblings to name a few. Fundamentally, thishypothesis addresses the different types of microorganismsyou would have been in contact with asa child. The larger this number is, the lower yourchances of developing an allergy. In order to developtolerance to antigens as a child, it is important to beexposed to diverse microorganisms.

The last major hypothesis is the ‘Vitamin Dhypothesis’, an interesting concept that has grown fromnumerous observations. Firstly, there are higher ratesof food allergy and hospital admissions for food-relatedanaphylaxis in children that live further from the equatorthan those that live nearer the equator. Secondly,children born in autumn or winter are more likely todevelop allergies compared to those who were born inspring or summer. Following these observations, studieshave shown that vitamin D has an important role inregulating several different immune functions.

Rather unsatisfyingly, it seems that allergiesdevelop due to a number of different factors. It ishoped that in the future through further genetictesting and observational studies that there will be aclearer idea of what causes allergies.

by Isabella Munford