Substances that cause allergy – ‘allergens’

• Food Allergy
• Insect venom allergy bee sting/wasp sting
• Drug allergy
• Pet dander
• Latex allergy
• Hay fever or allergic rhinitis
• Oral allergy syndrome
• Angioedema
• Urticaria
• Food intolerance
• Asthma
• Eczema
• Food and exercise induced anaphylaxis
• Idiopathic anaphylaxis
• Food protein induced enterocolitis syndrome (FPIES)

Food Allergy

Food allergy is common in very young children with 1 in 20 toddlers affected. The incidence in older children is much lower because children tend to grow out of their allergies. It is not known why food allergies develop but they occur more commonly in children with eczema or a family history of allergy. Children are most commonly allergic to a small group of foods, these account for 90% of food allergies in children. Allergy to other foods is less common, but the pattern is changing with allergies that were rare a few years ago becoming more common (e.g. kiwi and pulses).

The majority of children with food allergy have mild reactions; symptoms include lip swelling and urticaria. Even though the reactions are not life threatening they are distressing and food allergy may also significantly exacerbate eczema. Eczema is very common in food allergy and usually appears before the food allergy becomes apparent.

Food allergy has a significant impact on children and families for several reasons. There is a continuous fear of suffering allergic reactions. It becomes harder to provide a safe and nutritious diet for the family once a food allergy is diagnosed. Shopping becomes more time consuming, as all labels have to be checked for allergens and labels that inform of the risk of cross-contamination of purchased foods are confusing. Cross contamination means a food contains traces of an allergen, such as nuts, despite not containing nuts as an ingredient because the factory makes or packages nut products and nut proteins could contaminate the product. Similarly eating out at other people’s houses or in restaurants becomes a source of stress and requires planning. As a consequence the quality of life of affected families is adversely affected. The effect on activities will depend partly on whether the child reacts to minute traces of a food making reactions much more difficult to avoid and whether they have more than two food allergies. More than two food allergies significantly increase the difficulty of providing a safe and nutritious diet. It increases the risk of reactions and research shows it has a much greater impact on life than one or two food allergies alone. Nuts and peanuts count as one allergy in this context.

Table 2 in the link below provides information about the risk of anaphylaxis associated with certain foods and with environmental contamination.

For more information see Types of food allergy

Insect venom allergy bee sting/wasp sting

Anaphylaxis after insect stings does occur but is rare in childhood. Desensitisation therapy for this type of allergy is available from allergy specialists, but is rarely required. This type of treatment can either prevent further reactions after an insect sting or reduce the severity of any reaction experienced. Children who have large local reactions after an insect sting are not at significant risk of future anaphylaxis. An allergy specialist will be able to provide a risk assessment.

Drug allergy

Medicinal drugs may cause allergic reactions, most commonly these are mild rashes and anaphylaxis occurs rarely. Allergy to penicillin drugs is commonly claimed, but when investigated by a specialist, allergy is often not present.

Pet dander

Allergy to cats and dogs may cause worsening asthma control over time, which may be resolved by re-housing the pet. Rarely, a child may suffer a severe allergic reaction when visiting another house with a pet but once the diagnosis is made, this circumstance should be straightforward to avoid. Repeated severe allergic reactions may occur rarely in circumstances where avoidance is not possible (e.g. when a parent’s career depends on animal contact). In this case extra care is required by the parent to change clothes and shower on arrival to the home.

Latex allergy

This condition is now rare in children due to the increasing use of non-latex products in healthcare. Affected children most commonly develop mild reactions with urticaria and swelling after contact and anaphylaxis is extremely rare. Latex allergy is often blamed when children have otherwise unexplained reactions, usually because there has been a suspicion of latex contact from the environment. However, allergy testing should be performed as this association is often shown not to be genuine. The child should be assessed by an allergy specialist before this diagnosis is accepted.

Hay fever or allergic rhinitis

Rhinitis (commonly known as Hay fever) is usually caused by an allergic reaction in the airway and eyes to airborne plant allergens such as grass or tree pollen. This causes seasonal symptoms with worsening in the Spring and/or Summer. Allergic rhinitis can also be stimulated by ‘perennial’ allergens such as house dust mite or pet dander and in this case the symptoms are present all year round. Non-allergic rhinitis is also common and in this case there is no allergen to blame but the symptoms are identical. Symptoms include sore itchy, red watering eyes, nasal itching, running and blockage. These can be particularly troublesome, resulting in poor sleep, lethargy, missed school days and reduced examination performance. Hay fever itself is not life threatening but it is often associated with asthma. Allergic rhinitis may worsen asthma control and indirectly contribute to a severe allergic reaction. Allergic rhinitis in the Spring, stimulated by tree pollen is also part of the ‘oral allergy syndrome’ (see below).

Oral allergy syndrome

Oral allergy syndrome is a condition that occurs in people with allergic rhinitis to tree pollen (causing spring hay fever). People with this condition develop allergic reactions to certain fruits. Symptoms include lip swelling after eating the fruit, but cooked fruit is usually tolerated. In oral allergy syndrome reactions are usually limited to lip swelling and progression to difficulty breathing or collapse is uncommon. However, a related allergy to fruits is increasingly recognised in childhood where reactions can be severe, characterised by throat tightening and respiratory difficulty (e.g. to kiwi or peach) and there is a risk of anaphylaxis, although usually the trigger is easy to avoid.

Angioedema

Angioedema is a name used for swelling of deep tissues anywhere on the body, but it most commonly affects the face, the hands and the genitals. Swelling of these areas is not associated with redness like the more superficial swelling seen with hives/urticaria, which also involves swelling of the skin. In practice, the term angioedema is mainly used to describe dramatic swelling of the face. Angioedema is one symptom of allergy but does occur in other conditions where allergy testing may be negative.

Hereditary angioedema is a life-long condition caused by an inherited deficiency of the enzyme ‘C1 esterase inhibitor’. There is usually a family history of an affected member and the disease usually presents in the second decade of life with spontaneous angioedema or swelling (urticaria does not occur, as a rule). Swelling can appear spontaneously or after injury or surgery (e.g. tooth extractions). Angioedema can occur anywhere but may be life threatening if it affects the upper airway. Swelling in the bowel wall can cause repeated episodes of severe abdominal pain. Allergy tests are negative in this condition; diagnosis is by a specialised test for the defective enzyme in the blood, which is either reduced or poorly functioning. Treatment of acute episodes is with infusions of fresh frozen plasma or concentrated enzyme replacement.

Urticaria

Urticaria is the name given to the itchy raised skin rash otherwise known as hives, and is often described by patients as similar to a nettle sting rash. Urticaria has many causes. It is a common symptom of food allergy or may occasionally be caused by contact with grass in pollen-allergic children. Allergy tests will usually be informative. There is a distinct condition known as chronic urticaria and angioedema in which the child suffers repeated episodes of rash and swelling which appears identical to an allergic reaction (each episode typically lasts longer than 24hrs) but there is no allergic cause. It usually arises in a child with an insignificant history of allergy. Sometimes patients identify a physical trigger such as infection, heat, cold, sweating or exercise. Allergy tests in this condition are usually negative and so the diagnosis relies on exclusion of other allergies by a specialist.

Food intolerance

Food intolerance is any reaction to a food not related to allergy. Food intolerance may be caused by absence of an enzyme needed to digest the food – e.g. lactase deficiency means the enzyme lactase used to digest lactose, a milk protein, is absent and drinking milk causes diarrhoea. It may also be caused by a substance present in several foods for example sulphites in wine might exacerbate asthma in susceptible people. There are many different types of food intolerance, which may cause significant and disabling symptoms when the foods are eaten but reactions to traces of foods and immediately life-threatening reactions to such traces do not occur.

Asthma

Asthma is often caused by allergy in children. You should consult the guidance on asthma if asthma alone is claimed. If asthma is the only symptom of the allergy and there is no risk of anaphylaxis just use the asthma guidance to assess the case.

Eczema

Eczema is sometimes caused by allergy in children. If eczema is a symptom of the allergy and there is no risk of anaphylaxis just use the eczema guidance to assess the case.

Food and exercise induced anaphylaxis

In this uncommon condition, a food which is normally tolerated (usually wheat or shellfish) can cause anaphylaxis if the patient exercises within four hours after ingestion of the food. The exercise is usually out of the ordinary but may in fact be relatively mild. Avoidance of exercise for four hours after eating is the only way to prevent reactions. This condition usually requires allergy specialist diagnosis.

Idiopathic anaphylaxis

Idiopathic (of no known cause) anaphylaxis is a rare condition where the child undergoes episodes of spontaneous anaphylaxis - there is no allergic trigger. This is a frightening condition as there are no clear proceeding triggers, which the patient can avoid. The diagnosis is by exclusion of allergic and physical triggers and should be made by an allergy specialist.

Food protein induced enterocolitis syndrome (FPIES)

This rare syndrome usually begins in infancy. Affected children develop severe diarrhoea after eating any food protein, as a consequence they rapidly lose fluid from their circulation and present with shock. The appearance can be of anaphylaxis although the mechanism is unknown. Allergy tests are usually negative. Great care is required to avoid sources of food protein in the diet. The condition can resolve in early childhood.