An allergy occurs when the body's immune system over-reacts to normally harmless substances (called allergens). These substances may be in the air or what you touch or eat. Allergies are a contributing factor to conditions such as hay fever, asthma and eczema.
If you are an 'allergic' person, and you come into contact with an allergen, your immune system produces a special kind of antibody (IgE). Other cells release further chemicals such as histamine that cause the symptoms you experience.
The main environmental allergen is the dust mite - or, to be more precise, its faeces. Pollen, particularly from grass, trees and weeds, is another common allergen, as is animal dander (skin scales or flakes from the fur or feathers of animals).
A number of foods can also cause allergy: the most common are peanuts, dairy, eggs and seafood. However, true food allergies are not common and most reactions to food are more likely to be food intolerance rather than an allergic reaction that involves the body's immune system. Some moulds, and insect bites and stings can also cause allergies.
Cigarette smoke is often considered a cause but it is actually an irritant rather than an allergen. That means it does not cause the allergy, but makes an existing allergy worse.
Symptoms depend on which part of the body is affected. For example, hay fever (also known as seasonal allergic rhinitis, because it is mainly triggered by pollen) affects the eyes and nose, causing sneezing, a runny nose, watery, itchy eyes, irritated and itchy throat and, sometimes, a stuffy, blocked nose.
Perennial allergic rhinitis (a condition that causes symptoms all year round) often causes a stuffy, blocked nose.
Eczema (also called dermatitis) can affect the skin causing itchy, red rashes. Allergic contact dermatitis (a condition which is caused by the skin coming into contact with an allergen, such as nickel) is characterized by red, scaly skin that itches where it has made contact with the allergen. Asthma affects the respiratory system causing wheezing (a whistling sound in the chest), breathlessness, chest tightness and a cough.Allergies to some foods, bites or stings can cause urticaria (itchy blisters and red patches on the skin).
Severe allergic reactions
A very severe allergy can cause anaphylaxis, where the person can become flushed, break out in a widespread itchy rash, have difficulty breathing, suffer a severe drop in blood pressure, and eventually lose consciousness. This is a life-threatening situation and needs urgent medical attention (call for an ambulance and tell a member of the emergency services what is happening so they can bring the appropriate treatment).
What causes allergies?
While you do not inherit an allergy directly, you may inherit a tendency to be allergic. Doctors call that being atopic. Allergies start only if you are then exposed to an allergen (things that trigger an allergic response in the body).
The first year of life is crucial when it comes to setting off allergies in those with a genetic predisposition. For example, a child who's been exposed to lots of house dust mite allergen in their first year has a higher chance of developing asthma later in childhood compared to a child who hasn't been exposed to such high levels of the allergen.
Once you develop sensitivity to an allergen, an allergic response is set off again every time you are exposed to it.
There are two types of allergy test: a blood test (known as RAST, radioallergosorbent test). This identifies exactly what you are allergic to through a blood sample.
Skin prick test (the procedure we use). Here a needle is pricked into the skin through a drop of the suspected allergen, usually on the forearm or back. The size of the wheal and surrounding redness on the skin indicates how strongly you are allergic to a particular allergen.
There is another blood test (IgE) which checks the number of allergy cells circulating in the blood. This tells how allergic you are but not what you’re allergic to.
Skin prick testing has many advantages over RAST testing. For example: the results are available within 20 minutes whereas RAST results take days. We can test for a wider range of substances using the skin prick method than with RAST.