There are 4 sources of allergens/provocants:   Biological: food and drink; infections; animal and plant proteins (examples: nuts, cat fur, dust mites and pollen) Chemical: any of the 60,000 chemicals in common use Physical: radio and microwaves, infrared, visible and UV light, X-rays, atomic radiation and sound Mental: stress and lack of sleep

Contact with an allergen (the item that causes the reaction) can be with your skin, eyes, or with the lining of your lungs, mouth, oesophagus (gullet), stomach or intestine.

If you are allergic or sensitive to a food or chemical, then exposure will give you symptoms. For example, if you are allergic to perfume and you visit the perfume section of a department store, you may experience symptoms such as flu-like illness, palpitations, headache or diarrhoea. These symptoms may occur instantly or be delayed. Delayed responses are common and the delay often makes it difficult for the allergy sufferer to recognise the cause of the reaction.

Allergies can produce an amazing diversity of symptoms and can affect any system of the body. Allergies may in fact produce underactivity or overactivity of an affected organ or system, causing the body to work inadequately.

Substances that cause Altered Reactivity are called allergens or provocants. Common allergens are pollen or dust. Allergens may cause illness in a susceptible person and this resulting illness is often called an allergy.

To be allergic or sensitive, you must first have been exposed to an allergen. On that first encounter, your body’s immune system determined that the allergen was a threat to its wellbeing. The self-protecting response of the immune system was to produce an antibody. An antibody is a protein manufactured by the white blood cells to neutralise the allergen.

The next time you are exposed to the allergen, your antibodies respond. Allergic symptoms are the result of the reaction between the allergen and antibodies. In many cases the damage from the allergy does not go away when the symptoms stop. A malfunctioning immune system gives rise to allergic symptoms because of the production of highly reactive molecules called free radicals, which damage tissues.
Allergies are much more common after a trigger event such as an infection (especially viral infection), massive exposure to chemicals or radiation, or a shock, accident or bereavement. Alternatively, allergies may be started by long-term exposure to a provocant, often at a lower concentration level than the body accepts as a danger level.

Allergies are becoming much more common because of our modern way of living. Probably at least half of all patients visiting their general practitioners have symptoms wholly or partly attributable to allergy and/or their environment. The majority of these can be made better by using the management techniques we use at Breakspear Hospital.

An example of a modern common allergy is allergic rhinitis, commonly called hay fever. It was first described at the beginning of the Industrial Revolution by Dr John Bostock, who described it in himself. It took him several years to identify a dozen or more hay fever sufferers. This shows that it was quite uncommon before the Industrial Revolution.

Today hay fever is twice as common in towns as in the country. This is largely as a result of road traffic pollution and the effect of sunlight on traffic pollution, which is referred to as "photochemical smog". The chemical sensitivity that precedes triggering by pollen is increasingly being recognised and can also be treated.

Allergy occurs either when the total load of allergens is too high and overwhelms the abilities of the immune system to cope, or when one particular provocant to which the body is particularly susceptible is at a dangerous level for that person. (This is further explained in the section The Load Phenomenon.) There is enormous individual variation in susceptibility to an allergen. One level of a substance that produces no symptoms in one person may make another, more susceptible, person very ill.