Allergies: The right reaction

Learning objectives

After reading this feature you should be able to:

• Understand the widespread and growing prevalence of allergies in the UK
• Help patients and caregivers understand the nature of allergy
• Recognise the psychological impact of allergies on sufferers.

Allergy UK estimates that 44 per cent of British adults live with at least one allergy.

Between 10 and 15 per cent of children and 26 per cent of UK adults have allergic rhinitis (AR), says the charity. Prevalence is thought to have  risen by a third in the past 20 years. Meanwhile, up to 10 per cent of adults and children live with food allergies, and one in 11 children and one in 12 adults are treated for asthma.

Given that allergies and hypersensitivity disorders are so common, it is perhaps surprising that care is often imperfect. NICE has noted, for example, that “emollients [for eczema] are typically under-prescribed and under-used. This results in suboptimal treatment”.

A recent Lancet paper commented that highly effective therapies have helped to drive a marked improvement in asthma (including allergic asthma) morbidity and mortality in the past 15 years. Nevertheless, the review added that “undertreatment is still common”.¹ So, given the huge number of patients with allergies, how can pharmacy teams improve outcomes?

A good place to start is by helping patients and caregivers understand the nature of allergy.

Essentially, allergies arise when an allergen enters the body. A complex comprising an IgE antibody bound to its high-affinity receptor on mast cells and basophils captures the allergen.
The complexes then cross-link. This activates mast cells and basophils, which release the inflammatory mediators that underlie allergic symptoms.²

Allergic reactions arise within minutes of exposure to an allergen, so they differ from other immune responses, which typically take longer. The rapid onset seems to reflect the particularly high affinity between IgE and its receptor, and mast cells’ exquisite sensitivity: very few receptors on mast cells need to bind the allergen to trigger ‘immediate hypersensitivity’ reactions.²

Allergies arise in any tissues populated by IgE and mast cells, such as the respiratory tract, eyes, skin and gastrointestinal tract.² In people with allergic asthma, allergens trigger bronchospasm causing symptoms such as wheeze and breathlessness. The allergic response promotes airway inflammation, hyper-responsiveness and mucus hypersecretion. These changes contribute to asthma’s hallmark: variable airflow obstruction.¹ But this isn’t the whole story.

Inflammatory mechanisms

Several inflammatory mechanisms contribute to asthma.¹ For example, between 40 and 70 per cent of asthma patients show an immune pattern called type 2 inflammation, often driven by IgE and characterised by large numbers of eosinophils in the tissues.³

Type 2 inflammation evolved to counter infestations of parasitic worms. Asthma patients with eosinophilia tend to respond to inhaled corticosteroids⁴ but inflammation driven by neutrophils seems to underlie some cases of asthma in people with low levels of type 2 inflammation and steroids are typically less effective.⁴ After checking adherence and inhaler technique, patients with poorly controlled asthma should be referred.

As allergies arise in any tissue populated by IgE and mast cells, many people are allergic to more than one allergen. Allergy UK estimates that 48 per cent of British adults with allergy react to more than one allergen. For instance, some 80 per cent of people with asthma have AR. A study of 94 children aged between 10 and 14 years with food allergies reported that they reacted to, on average, three food allergens, although this varied from one to 11 (see Figure 1).⁵ A third (36.2 per cent) of the 94 children experienced an allergic reaction and 8.5 per cent had to use their adrenaline autoinjector in the six months before the study.⁵

Nature and nurture

Allergies arise from the interaction between each patient’s genetic make-up and the environment. For instance, alterations in skin bacteria (microbiota) can trigger atopic dermatitis (AD) in genetically susceptible people.⁶ About 80 per cent of people with eczema harbour Staphylococcus aureus in their lesions (skin affected by AD).

Japanese researchers found that 45 per cent of infants at one month and 49 per cent at six months of age were colonised with S. aureus. Previous studies had suggested that S. aureus colonises the healthy skin of 5 to 10 per cent of people older than two years of age.⁷

Skin colonisation with S. aureus at six months of age increased the risk of developing AD at one and two years of age by about five-fold. At one and two years of age, 24.8 and 24.0 per cent respectively of children colonised with S. aureus developed AD.⁷

Researchers are actively investigating the role of the microbiome and the extent to which pro- and prebiotics help prevent AD and other allergies, with some encouraging results. However, further studies are needed.⁸

Eczematous lesions and intense itch (pruritus) are AD’s most common symptom and sign respectively.⁹ Eczema causes lighter skin to become itchy, inflamed and red. Darker skin affected by eczema appears itchy, brown, purple, grey or ashen. These visible changes hide marked skin damage and inflammation in the skin’s upper layer (stratum corneum) that contributes to itch.

Scratching is hard to resist but can lead to scarring and lichenification as well as releasing more proinflammatory mediators that exacerbate eczema and xerosis (dry skin).^10,11 Atopic dermatitis, however, is more than skin deep: there is a two-way relationship between allergies and the mind.