Allergic disease: The basics

• Incidence
• Predisposing Factors
• Natural History
• Clinical History
• General Investigations
• Prognosis
• References

Incidence

Approximately 20% of the general population is atopic, with a number of these people suffering from allergic diseases such as eczema, urticaria, allergic rhinitis, asthma or anaphylaxis.⁴ Approximately two-thirds of atopic individuals have clinical allergic disease.²

Predisposing Factors

Allergic reactions, or atopy, have a significant genetic predisposition. The risk of passing on the atopic trait to children is 75% with 2 atopic parents, which reduces to 50% and 15% with 1 or 0 atopic parents respectively.² The nature of what, exactly, is inherited by atopic individuals and what predisposes to allergy is complex, and involves several gene-environment interactions.² The genetic basis is polygenic and may include regions such as certain HLA haplotypes, as well as non-MHC genes such as the IgE receptor on chromosome 11. ⁴

Natural History

There are several steps in the development of an allergic reaction:³

1. Sensitisation.
   • Previous exposure to a substance results in B lymphocytes making specific IgE against that substance. These IgE molecules attach to receptors on mast cells;
2. Activation of mast cells.
   • When the substance again comes in contact with IgE on the mast cell, 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 result in inflammation, increased vasodilation, increased capillary permeability resulting in oedema, redness and bronchoconstriction.

Clinical History

Allergic diseases, such as eczema, hay fever and asthma, are usually diagnosed on the basis of the clinical history.
Allergic symptoms may be perennial – experienced all year round – or seasonal, when they only occur at certain times of the year (e.g. spring). This can give an indication of the allergens involved. For example, people allergic to pollens often experience allergies that are worse in spring, whereas allergies to house dust mites and animals are often experienced all year long.

General Investigations

There are several investigations that can be useful in patients with allergic disease:²

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 or 100% sensitive or specific, 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.

Prognosis

The tendency to develop allergic reactions, or atopy, is usually lifelong. However, the clinical manifestations of allergy often change over time. For example, the concept of the ‘allergic march’ has been developed as a result of research suggesting that children often go through a sequence of allergic disease, developing allergic eczema in early infancy, followed by allergic rhinitis and allergic asthma, usually by 5 years of age.¹

References

1. Gold MS, Kemp AS. Atopic disease in childhood. Med J Aust. 2005;182(6):298-304. [Abstract| Full text]
2. Peakman M, Vergani D. Basic and Clinical Immunology. New York: Churchill Livingstone; 1997. [Book]
3. Solomon EP, Berg LR, Martin DW. Biology (5th edition). Fort Worth, TX: Saunders College Publishing; 1999. [Book]
4. Warrell DA, Cox TM, Firth JD, Benz EJ Jr. Oxford Textbook of Medicine (4th edition). Oxford: Oxford University Press; 2003. [Book]