From a simple runny nose, itchy eyes and sneezing to potentially fatal asthma and food allergies, millions of people suffer from allergies. What does this tell us about the body's immune defences, and is it possible to stop allergies from developing?

What causes allergies?

Allergies are caused by a damaging response of the immune system to an otherwise harmless feature of the environment.

Allergic symptoms include sneezing, wheezing, itching skin and eyes and violent reactions to food, nearly all as a result of the same immune response. Pollen, cat fur, house dust, peanuts and other substances, called allergens, are only the trigger. The problems are caused by the immune system.

Allergy was first described as long ago as 1906 by an Austrian doctor, Clemens von Pirquet. He defined it as "a specific acquired altered reactivity which follows initial exposure to a foreign protein". The body reacts differently on its first exposure to an allergen to the way in which it does on subsequent exposure, and this description fits all normal immune reactions. The difference between a normal immune reaction and an allergy is the outcome.

Our body is very successful at defending us against attack. The entire external surface of our body is covered with a continuous cell layer called epithelium which keeps out most infectious agents, called pathogens, and other foreign particles.

On the inside of the body there is an army of immune cells which challenge anything which does cross the barrier. The first time a particular pathogen infects the body it takes some time to be fought off, during which time you are ill. But the immune system has a memory. When the pathogen is encountered again, the immune system is ready for it. Later encounters are met with a much more rapid and effective immune response. This is the basis of immunity.

The first time a potential allergen is encountered, you experience no symptoms. But, in spite of this, the immune system is silently reacting as if a pathogen were present, memorising it for subsequent encounters.

Allergic versus non-allergic

In non-allergic people, the immune system has already identified the potential allergens as harmless, and does not respond the next time it meets them.

Those who are allergic, however, have an immune system which immediately attacks allergens, reacting as though they are pathogens. This is what leads to the symptoms of allergy.

In the developed world about 30 percent of the population are affected by allergies. In Europe, hay fever affects between 10 and 20 percent of the population, and eczema between 10 and 12 percent. Of the children who have eczema, up to one in five will develop asthma in later life. In Britain, asthma affects between 10 and 15 percent of children, and five percent of adults.

However, throughout the developed world, the number of people with allergies has been steadily increasing over the past 20 to 30 years.

Why is this happening? The answer is that no-one is entirely sure, but there are many possible reasons. The development of allergic disease depends on genetics and the environment, and allergies tend to run in families. The genetic predisposition to allergies is called atopy, and the activity of the genes for atopy is governed by the environment. There is plenty of research being carried out in the Western world on genes which may be candidates as atopy genes, with particular attention being focused on asthma.

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