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;
- Foods including tree nuts, peanuts, shellfish, fish, milk, eggs, wheat and more;
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;
How is it Diagnosed
There are several investigations that can be useful in patients with allergic disease:
- 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.
- 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.
treatments are often tailored to the specific allergic disease, particularly eczema, allergic rhinitis and asthma. There are, however, some general treatment principles:
- 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)
- 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.
- 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.