Immunologist Dr. Michael Levi once described an allergy as being like a phobia of the immune system. It's as if our bodies take a dislike to a substance and panic whenever it is around. 

In short, an allergy is an overreaction of the immune system to a harmless substance.

The word allergy literally means altered working. It was first used at the beginning of the 20th Century when it was observed that dogs endured a fatal reaction if they were immunised using proteins from a different animal.  Instead of experiencing 'prophylaxis', the protection usually provided by the procedure, the dogs suffered the opposite - a pathological response that became known as 'anaphylaxis'.

The first official definition by Von Piquet in 1906 described allergy as a specifically changed reactivity of the host to an agent on the 2nd or subsequent occasion.  This statement, however, also describes the beneficial immune development known as Acquired Immunity, which is highly protective to the host, providing protection against repeated exposure to the same virus.

Allergies are purely and unnecessarily pathological in nature, so whilst not incorrect this description was deemed too broad and the definition of Type I Hypersensitivity, or Anaphylaxis, was adopted.

When an immune response is exaggerated or inappropriate to the stimulus and leads to pathological events within the body it is called hypersensitivity.

There are two types of immunity: cellular immunity - immune cells called macrophages calmly engulfing and eliminating non-living, low-risk invaders; and humoral immunity - the attack and destruction of living cells. The humoral immune attack often involves 'explosion' of cells, releasing their contents and leading to characteristics typical of inflammation such as redness, swelling and irritation.

An allergic reaction is as if the immune system had made a mistake and launched a humoral immune attack against a non-living, harmless foreigner, such as pollen or dust-mite excrement, leading to inflammation-like symptoms.

What exactly is a hypersensitivity reaction?

There are 4 types of hypersensitivity reaction, defined according to their mechanisms and manifestations.

Type I hypersensitivity reactions, also referred to as 'anaphylaxis', occur in sensitised people when a substance they are allergic to, an allergen, enters the body. This causes the release of immune proteins called immunoglobulin E (IgE), which stimulate immune cells called mast cells to release active substances such as histamine. This chain of events leads in the local area to the well-known symptoms of inflammation associated with allergies, i.e. irritation of the eyes and nose in hayfever, constriction of the airways with asthma, and the redness of hives.

In severe cases a generalised reaction known as anaphylactic shock can occur. This can be life threatening and usually happens as a result of a food allergy, or bee or wasp sting.

While all 'true' allergies are Type I Hypersensitivity reactions, the term 'allergy' is often used to describe a whole host of clinical conditions, not all of which fit the description. Some food or drug allergies, for example, are actually of a delayed type, or Type IV, hypersensitivity reaction but are often clinically referred to as allergies for simplicity.

What causes allergies?

Any material that can trigger an allergic response in a sensitised individual is called an allergen. Allergens are usually non-living materials that are commonly found all around us, such as pollen from trees or grasses, droppings of house dust mites, and animal dander. Once a person becomes sensitised to a particular substance, each subsequent exposure will cause an allergic response.

How does a person become 'sensitised'?

In the last two decades the prevalence of allergic diseases has risen fourfold, yet it is still not clear how or why a person develops an allergy. Family history appears to be predictive of allergy development, hinting at a genetic component, while contact with allergens in childhood also seems able to 'prime' individuals.  Unlike with viral infections, where early exposure is beneficial, over-exposure to allergens in the first year of life has been shown to promote allergic disease later. Cigarette smoke, while itself not an allergen, appears not only to contribute to the symptoms of some allergies (e.g. asthma) but is also thought to promote their development.

Many allergic individuals are atopic, which means that they are genetically predisposed to overproduce IgE. This makes them more likely to suffer from allergic diseases such as asthma and eczema, both of which often occur together and tend to run in families. 50% of asthmatics for example also suffer with eczema.

Home comforts like central heating, double-glazing and soft furnishings mean that allergens are becoming an increasingly unavoidable part of our environment, providing the perfect conditions for house dust mites to accumulate.

Who can be affected?

Anyone can be allergic. Although many allergies such as eczema and asthma are lost with childhood, some, particularly hayfever, continue into adulthood, and it is not un-heard of for people to develop new allergies even into their 60's. Thankfully treatments for allergies are advancing all the time, and it is now easier than it has ever been to manage the symptoms.

To see what our readers suggest, look at the Allergies message board, an interactive area of the site where you can share your advice and experiences.

03/06/2009