What is Allergic Disease

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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.

Probable Outcomes

The tendency to develop allergic reactions is usually lifelong, however the manifestations and symptoms can change over time. Children often go through a sequence of allergic diseases, developing allergic eczema in early infancy, followed by allergic rhinitis and allergic asthma, usually by 5 years of age.

Different people experience different types of reactions which may change over time, and may improve or get worse depending on a number of factors, which can include where they live and their environment.

How is Allergic Disease Diagnosed?

There are several investigations that can be useful in patients with allergic disease:

Blood Tests:

* Total serum IgE levels are elevated in the majority of patients with allergic disease.
* Serum eosinophilia is often observed in patients with allergic disease.

These blood tests are not diagnostic, but they can be useful to determine the severity of the allergic tendency.

Tests to determine allergies:

* Skin prick tests against a wide variety of allergens are highly sensitive and are the first line test to determine what allergens a patient is allergic to.
* Radioallergosorbent tests (RAST) detect serum levels of allergen-specific IgE. It is used when the history and skin prick test results are conflicting, when skin prick testing cannot be performed, or when desensitisation is being considered.

How is Allergic Disease treated?

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Specific allergy treatments are often tailored to the specific allergic disease, particularly eczema, allergic rhinitis and asthma. There are, however, some general treatment principles:

Allergen avoidance

* Avoiding areas where allergens are in high concentration (e.g. the park in springtime where heyfever causing pollens are in high concentration, or avoiding cats or dogs)
* Reducing exposure to allergen (e.g. reducing exposure to house dust mite allergen by encasing mattresses and bedding in dust mite protectors, regular vacuuming)

Allergy medications

* Anti-inflammatory medications designed to reduce the overzealous immune responses seen in allergic disease, such as topical corticosteroids (inhaled, intranasal, skin creams) have a role in eczema, allergic rhinitis and asthma.
* Antihistamines have a role in eczema and hayfever, but not in asthma.
* Adrenaline has a role in severe allergic reactions such as anaphylaxis.

Allergy desensitisation:

* Desensitisation is not suitable for all allergic diseases. It is particularly useful for insect stings and drug allergies, where it is usually effective, but less useful for asthma or allergic rhinitis, where it is less effective. It is most useful where there are a small number of allergies, such as only insect stings, and less useful when there are multiple allergies.

For more health information on allergy treatment visit your local doctor.

Allergic Disease References

1. Gold M, Kemp A. Atopic disease in childhood. MJA. 2005; 182(6): 298-304.
2. Peakman, M. Vergani, D. Basic and Clinical Immunology. 1997. Churchill Livingstone. New York.
3. Solomon, E. Berg, L. Martin, D. Biology. 5th ed. 1999. Saunders College Publishing. Fort Worth.
4. Warrell, D. Cox, T. Firth, J. Benz, E. Oxford Textbook of Medicine. 4th ed. 2003. Oxford University Press. Oxford.

Tags: Allergic Disease

This entry was posted on Wednesday, January 21st, 2009 at 6:20 am and is filed under Allergy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.