Allergy

Published on 21/03/2015 by admin

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Last modified 21/03/2015

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Allergy

An abnormal immune system may result in:

• Allergic diseases

• Immune deficiencies

• Autoimmune disorders – either organ specific (e.g. type I diabetes mellitus) or systemic (e.g. systemic lupus erythematosus).

Paediatric allergy

Allergic diseases which affect children include food allergy, eczema, allergic rhinitis and conjunctivitis, asthma, urticaria, insect sting hypersensitivity and anaphylaxis. The reasons allergic diseases are important are:

• In the UK up to 40% of children have allergic rhinitis, eczema or asthma and up to 6% develop food allergy

• They are increasing in prevalence in many countries

• They are the commonest chronic diseases of childhood and the commonest cause of school absence and acute hospital admissions

• They cause significant morbidity and can be fatal, with about 20 children dying from asthma and 2 from food anaphylaxis in the UK each year.

Explanations of some of the terms used in ‘allergy’ are listed in Box 15.1.

Box 15.1 Allergy definitions

• Hypersensitivity – objectively reproducible symptoms or signs following exposure to a defined stimulus (e.g. food, drug, pollen) at a dose which is tolerated by normal people.

• Allergy – a hypersensitivity reaction initiated by specific immunological mechanisms. This can be IgE mediated (e.g. peanut allergy) or non-IgE mediated (e.g. coeliac disease).

• Atopy – a personal and/or familial tendency to produce IgE antibodies in response to ordinary exposures to potential allergens, usually proteins. Strongly associated with asthma, allergic rhinitis and conjunctivitis, eczema and food allergy.

• Anaphylaxis – a serious allergic reaction that is rapid in onset and may cause death.

Mechanisms of allergic disease

Many genes have been linked to the development of allergic disease. Polymorphisms or mutations in these genes lead to a susceptibility to allergy.

Allergic diseases occur when individuals make an abnormal immune response to harmless environmental stimuli, usually proteins. The developing immune system must be ‘sensitised’ to an allergen before an allergic immune response develops. However, sensitisation can be ‘occult’, e.g. sensitisation to egg from exposure to trace quantities of egg in maternal breast milk.

Only a few stimuli account for most allergic disease:

• Inhalant allergens, e.g. house-dust mite, plant pollens, pet dander and moulds in asthma and rhinitis and conjunctivitis

• Ingestant allergens, e.g. nuts, seeds, legumes, cow’s milk, egg, seafood and fruits in acute allergic reactions or eczema

• Insect stings/bites, drugs and natural rubber latex.

Proteins with an unstable tertiary structure may be rendered non-allergenic by heat degradation or other forms of processing. For example, some children are allergic to raw apples, but can tolerate eating cooked apples.

Allergic immune responses are classified as IgE mediated or non-IgE mediated. IgE-mediated allergic reactions have a characteristic clinical course:

• An early phase, occurring within minutes of exposure to the allergen, caused by release of histamine and other mediators from mast cells. Causes urticaria, angioedema, sneezing and bronchospasm

• A late phase response may also occur after 4–6 hours. This causes nasal congestion in the upper airway, and cough and bronchospasm in the lower airway.

The majority of severe life-threatening allergic reactions are IgE mediated.

Non-IgE-mediated allergic immune responses have a delayed onset of symptoms and more varied clinical course.

The hygiene hypothesis

It is not clear why the prevalence of allergic diseases has increased in many countries and the speed of this change suggests an environmental cause. A consistent observation is that the risk is lower in younger children of large families and in children raised on farms. These findings have led to the hygiene hypothesis, which proposes that the increased prevalence is due to altered microbial exposure associated with modern living conditions (Fig. 15.1). Although the hypothesis remains the leading explanation for the increase in allergic disease, it is mainly supported by indirect evidence.

Figure 15.1 Hygiene hypothesis.

The allergic march

Allergic children develop individual allergic disorders at different ages:

• Eczema and food allergy usually develop in infancy; both are often present

• Allergic rhinitis and conjunctivitis and asthma occur most often in preschool and primary school years

• Rhinitis and conjunctivitis often precede the development of asthma, and in children with asthma, up to 80% have coexistent rhinitis.

The presence of eczema or food allergy in infancy is predictive of asthma and allergic rhinitis in later life. The progression is referred to as the ‘allergic march’.

Prevention of allergic diseases

Many interventions have been tried to prevent allergic disease, or interrupt the allergic march. These include exclusive breast-feeding for at least 3–4 months (or if not possible, then use of hydrolysed formula instead of standard formula milk) to reduce the risk of eczema and cow’s milk allergy and the use of probiotics for eczema in infancy. Other approaches include altering allergen exposure (avoidance of allergens in early life, or alternatively, exposure to large doses of allergens to induce immune tolerance), prebiotics (non-digestible oligosaccharides naturally present in breast milk), nutritional supplements (e.g. omega-3 fatty acids, vitamin D, antioxidants, trace elements) and medications (e.g. antihistamines, immunotherapy). However, none have been shown, long term, to prevent children from developing allergic diseases.

History and examination

The child and family may not volunteer a history of allergic disease as they have come to consider the symptoms as normal, e.g. the child who coughs most nights or has a blocked nose most of the time may not perceive this as abnormal. As allergic diseases are multisystem, in addition to the signs of individual allergic diseases, examination may reveal:

• Mouth breathing (Fig. 15.2a). Children who habitually breathe with their mouth open may have an obstructed nasal airway from rhinitis, and there may also be a history of snoring or obstructive sleep apnoea

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