Dilation of small blood vessels

This can lead to hives, fluid retention and swelling, nasal blockage and headaches.

Smooth muscle spasm

This can produce contraction of the airways and gut spasm.

Increased secretions

Mucus flow and discharges increase and are manifest in hay fever, conjunctivitis, asthma and ear complaints.

It is the functioning of this mechanism that goes wrong in most allergic reactions. For some unknown reason, IgE antibodies are produced in response to harmless molecules. Any substance that triggers this abnormal reaction is called an ‘allergen’.

Further research into the immune system has shown that there are other mechanisms whereby other antibodies are involved in hypersensitive reactions, such as in delayed contact dermatitis, or when antibodies form immune complexes with invading molecules and are deposited from the bloodstream into tissues. However, the IgE mechanism is the principal one involved in allergy and the most important to understand.

Allergic reactions often happen immediately or within a few hours of encountering an allergen. If you are repeatedly in contact with an allergen, such as a commonly eaten food or house dust mites, you may have continual symptoms which occasionally worsen. You can also get late or delayed reactions, resulting from the on-going chemical changes in your body. These can happen up to several days later in the case of reactions to food.

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