Allergy basics

What are allergies?

Allergies are an abnormal response of the immune system. People who have allergies have an immune system that reacts to a usually harmless substance in the environment.  This substance (pollen, mould, dust, etc.) is called an allergen.

Allergies are a very common problem, affecting an estimated one in three of the UK population at some point in their lives.

What happens during an allergic reaction?

When a person is exposed to an allergen, a series of events takes place:

1. The body starts to produce a specific type of antibody, called IgE, to bind the allergen.
2. The antibodies attach to a form of blood cell called a mast cell. Mast cells can be found in the airways, in the gastrointestinal tract, and elsewhere.  The presence of mast cells in the airways and gastrointestinal tract makes these areas more susceptible to allergen exposure.
3. The allergens bind to the IgE, which is attached to the mast cell. This triggers a reaction that allows the mast cells to release a variety of chemicals including histamine, which causes most of the symptoms of an allergy, including itchiness or runny nose.

If the allergen is in the air, the allergic reaction will most likely occur in the eyes, nose, and lungs. If the allergen is swallowed, the allergic reaction often occurs in the mouth, stomach, and intestines. Sometimes enough chemicals are released from the mast cells to cause a reaction throughout the body, such as urticaria - itchy red welts, decreased blood pressure, shock, or loss of consciousness.

What are the symptoms of allergies?

Allergy symptoms can be categorised as mild, moderate, or severe ( anaphylactic).

• Mild reactions include those symptoms that affect a specific area of the body such as a rash, itchy, watery eyes, and some nasal congestion. Mild reactions do not spread to other parts of the body.
• Moderate reactions include symptoms that spread to other parts of the body. These may include itchiness or difficulty breathing.
• A severe reaction, called anaphylaxis, is a rare, life-threatening emergency in which the response to the allergen is intense and affects the whole body. It may begin with the sudden onset of itching of the eyes or face and progress within minutes to more serious symptoms, including abdominal pain, cramps, vomiting, and diarrhoea, as well as varying degrees of swellings that can make breathing and swallowing difficult. Mental confusion or dizziness may also be symptoms, since anaphylaxis causes a quick drop in blood pressure.

Does everyone have allergies?

No. Most allergies are inherited, which means they are passed on to children by their parents. People inherit a tendency to be allergic, although not to any specific allergen. When one parent is allergic, their child has a 50% chance of having allergies. That risk jumps to 75% if both parents have allergies.