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An allergy is an adverse reaction that the body has to a particular food or substance in the environment. Most substances that cause allergies are not harmful and have no effect on people who are not allergic. Any substance that triggers an allergic reaction is called an allergen.

Common allergens include pollen, house dust mites, mould and pets. Less common allergens include nuts, fruit and latex. An allergy develops when the body’s immune system reacts to an allergen as though it is a threat, like an infection. It produces antibodies to fight off the allergen, in a reaction called the immune response.

The next time a person comes into contact with the allergen, the body "remembers" the previous exposure and produces more of the antibodies. This causes the release of chemicals in the body that lead to an allergic reaction.

Common allergic disorders include asthma, eczema and hay fever. Symptoms of an allergy can include sneezing, wheezing, coughing and skin rashes. The nature of the symptoms depends on how you came into contact with the allergen. For example, you may experience problems with your airways if you breathe in pollen.

Anaphylactic reactions are those that cause a serious reaction that then affects the respiratory system and these can be life-threatening. The allergen could be ingested by eating it, inhaled by breathing it in, absorbed through the skin or injected like a bee sting. 

Allergic reactions are not just an anaphylactic type reaction. There are different levels of reactions which include allergies, sensitivity and intolerance.

An Allergy is a reaction produced by the body’s immune system when it encounters a normally harmless substance.

Sensitivity is the exaggeration of a normal side effect produced by contact with a substance. For example, the caffeine in a cup of coffee may cause extreme symptoms, such as palpitations and tremble, when it would usually only have this effect when taken in much larger doses.

Intolerance is where a substance (such as lactose or gluten) causes unpleasant symptoms such as diarrhoea for a variety of reasons but does not involve the immune system. People with an intolerance to certain foods can typically eat a small amount without having any problems. In contrast, people with a food allergy will have a bad reaction even if they come into contact with a tiny amount of the food to which they are allergic.

Where someone has an intolerance or sensitivity, treatment is often to remove the source and let nature take its course but where there is an anaphylactic reaction to an allergen, auto-injectors are often used to inject a pre-set dose of adrenaline into the muscle of the body which reduces the effect until emergency help arrives. Signs and symptoms of an anaphylactic reaction include:

  • Itchy skin or a raised, red skin rash
  • Swollen eyes, lips, hands and feet
  • Feeling lightheaded or faint
  • Narrowing of the airways which can cause wheezing and breathing difficulties
  • Abdominal pain, nausea and vomiting
  • Collapse and unconsciousness

Anaphylaxis should always be treated as a medical emergency. If you suspect that you or somebody else is experiencing symptoms of anaphylaxis, you should immediately dial 999 for an ambulance.