What is Allergy and anaphylaxis?
The focus of this guidance is severe allergy; this is defined as allergy with a risk of anaphylaxis (an acute hypersensitivity reaction). Allergy is a common condition characterised by a ‘hypersensitivity’ reaction or exaggerated sensitivity to substances that are normally tolerated. These substances are called allergens and examples include peanuts, milk and grass pollen. In allergy, the body has made an unwanted and harmful antibody called Immunoglobulin E or ‘IgE’. The interaction between the allergen and IgE antibody leads to symptoms that we call allergic reactions. Reactions can be mild – causing eczema to flare up or can be severe - anaphylaxis. Anaphylaxis may be brought about by other mechanisms in some people when allergy is not the cause and IgE antibody is not involved.
The commonest cause of anaphylaxis in children is peanut allergy. The symptoms occur because of the child’s immunological response to peanut protein. Common symptoms include swelling of the lips, throat and mouth within 30 minutes of eating peanuts. The most severe symptom of allergy is anaphylaxis. Definitions of anaphylaxis vary but one consistent feature is that it is a life threatening allergic reaction. Life is threatened by one or more of the following effects -:
- severe difficulty breathing due to throat swelling (laryngeal oedema)
- sudden severe asthma
- collapse due to low blood pressure
Any of these allergy related effects can reduce oxygen supply to vital organs and may result in death. The rate of death per anaphylactic episode in food allergy is 1 in 100 events. As a potentially fatal condition and one that rarely resolves, anaphylaxis can have a profound impact on quality of life, extending way beyond the acute phase of the illness. Although still often thought of mainly as an acute disorder, the long term burden of this diagnosis on the family and the impact on quality of life of carers should not be under-estimated.
Many other conditions may be claimed under allergy or labelled allergy. This guidance covers most of these.
Children with allergies frequently have more than one allergic condition. Eczema and asthma commonly accompany allergy with a risk of anaphylaxis. Separate guidance is provided on these conditions.
How does an allergy develop?
In order to develop an allergy a person must become ‘sensitised’ to a specific allergen. Sensitisation involves exposure to an allergen and in most normal people nothing would happen. When an allergy develops, abnormal IgE specific to that allergen is made. Once the specific IgE is produced, that person is then ‘sensitised’ to the allergen and is ready to react to it next time they come in to contact with it. However, not everyone with IgE becomes clinically allergic and other factors are involved, which are poorly understood. Common allergens include pollen, moulds, animals, dust mites and foods.
Allergies develop more commonly in children from families with a history of allergy in the parents or siblings. Certain allergies tend to develop commonly at particular ages. This process is known as the ‘allergic march’. For example, eczema often begins in infancy followed by food allergy in toddlers. Hay fever usually begins towards the end of the first decade of life. Exceptions occur and ‘adult-type’ allergies (e.g. hay fever, tree nut or shellfish allergy) are increasingly diagnosed in young children.
