Virtual Medical Centre

Allergic Disease (An Introduction)

• What is Allergic Disease?
• Who gets Allergic Disease?
• Predisposing Factors
• Progression
• Probable Outcomes
• How is Allergic Disease Diagnosed?
• How is Allergic Disease treated?
• Allergic Disease References

What is Allergic Disease?

Allergies occur when the immune system reacts against harmless substances in the environment. The immune system is extremely complex and so there are many opportunities for things to go wrong. When the immune system begins to react against harmless substances in the environment, this can lead to allergic reactions, which are exaggerated, damaging immune responses to substances that are normally harmless.

When people with allergy diseases are exposed to common environmental substances such as house dust mite or grass pollens, a type of white blood cell (B lymphocytes) produce specific antibodies known as IgE against that substance. This IgE then attaches itself to another type of white blood cell (mast cells), and when the mast cells come into contact with that substance again, they initiate a complex immune response that leads to the allergy.

Different people with allergies are allergic to different substances. Some of the substances that people are commonly allergic to include:

• House dust mite
• Pollens from grasses and trees
• Animal dander including cat, dog, horse
• Moulds
• Foods including tree nuts, peanuts, shellfish, fish, milk, eggs, wheat and more
• Latex

Different people with allergies also react differently when they are exposed to the substance they are allergic to. Some common reactions include:

• Allergic eczema or urticaria
• Allergic rhinitis (hayfever)
• Allergic asthma
• Anaphylaxis

For more health information on allergies, visit Anatomy and Physiology of Allergy

Who gets Allergic Disease?

Approximately 20% of the general population is defined as atopic, where they have a tendency to be allergic. These people will have a positive reaction to skin prick tests or RAST tests, but they may or may not suffer from any symptoms. Approximately two-thirds of atopic individuals will have clinical allergic disease, or allergic reactions to the substance that they are allergic to.

Predisposing Factors

The tendency to have allergic reactions, or atopy, is inherited. If you have 2 parents who are atopic (have a positive skin prick test), your risk of being atopic is 75%. The risk goes down to 50% if you have 1 atopic parent, and to 15% if neither of your parents are atopic.

Environmental factors also have a role in determining whether or not a person becomes atopic, and research is continuing to determine whether there is anything that can be done to prevent a person from become atopic.

Research is also continuing to determine why some people who are atopic develop allergic reactions while other people who are atopic remain asymptomatic (do not have any symptoms of allergic reactions).

Progression

There are several steps in the development of an allergic reaction.

1. Sensitisation

   Previous exposure to a substance results in B lymphocytes making specific IgE against that substance. The IgE then attaches to receptors on mast cells in the blood stream.

2. Activation of mast cells

   When the substance again comes in contact with IgE on the mast cell (such as when the allergen is inhaled), the mast cell releases histamine and other mediators (including serotonin and leukotrienes).

3. Effects of histamine and other mediators

   When these mediators are released, they lead to the symptoms of allergic diseases.