What allergy is and when to suspect it
An exaggerated immune response to substances that are harmless to most people. When symptoms recur in a particular setting, they are worth investigating.
What allergy is
Allergy is an exaggerated and inappropriate response of the immune system to substances that are normally harmless to most people, such as pollens, house dust mites, certain foods or animal danders.
In people with allergy, the immune system recognises allergens as a possible threat and triggers a specific immune reaction. In rapid-onset allergy this mechanism is known as type I hypersensitivity, or IgE-mediated allergy.
People with a genetic predisposition to develop allergic sensitizations are described as atopic. Over time, the allergic profile can change: new sensitizations can appear, symptoms can change, and the clinical picture can evolve from one stage of life to another.
The atopic march
The term allergic march, also called the atopic march, refers to the natural course of atopic manifestations, characterised by a typical sequence of immunoglobulin E mediated responses and clinical symptoms. These can appear early in life, can persist for years or decades, and often resolve spontaneously with age.
This concept is frequently misunderstood as a progression from minor symptoms or mild disease towards more severe chronic manifestations. It has also been wrongly interpreted as representing only the progression from atopic dermatitis in infancy to airway disease, particularly asthma at school age. Such interpretations have been shown to underestimate the variability and heterogeneity of how atopy develops during the first decade of life.
Numerous epidemiological studies, including non-interventional birth cohort studies carried out over the last two or three decades, have clarified the natural course of atopic disease in early life. The first decade of life proves to be a special and unique period compared with later ages, because the annual incidence of clinical manifestations and of the associated immune responses is far higher than at any other time of life.
The natural course and the sensitization process
In general, no clinical symptoms are detectable at birth, apart from dry skin. Although IgE production begins as early as the 11th week of gestation, no specific sensitization to food or inhalant allergens can be detected in cord blood using standard methods for measuring serum IgE antibodies.
The first IgE responses directed against food proteins can be seen in the first weeks or months of life. In every region of the world these are most frequently directed against proteins from hen’s egg and cow’s milk, regardless of feeding method, breastfeeding or formula.
These strong infant IgE responses to food proteins can be regarded as markers of atopic reactivity in general, since they have been shown to predict later sensitization to other food proteins, such as peanut or tree nuts, or to indoor and outdoor aeroallergens.
When to think about allergy
Allergy should be considered when symptoms appear in a particular exposure setting and tend to recur. In other words, not every sneeze, cough, rash or abdominal pain means allergy, but it becomes important to note when symptoms appear, how quickly they appear, how long they last and whether they recur with the same trigger.
Respiratory allergy
Suspicion arises particularly when the patient has repeated sneezing, watery nasal discharge, a blocked nose, nasal itching, red eyes or watering eyes in certain seasons, on contact with animals, dust or mould, or in particular indoor spaces.
Symptoms appearing in spring may suggest sensitization to tree or grass pollens, while persistent symptoms, particularly in the morning or at night, may point towards dust mites or indoor allergens.
IgE-mediated food allergy
Suspicion is higher when symptoms appear quickly, usually within minutes to about 2 hours after eating the suspected food.
Manifestations can include urticaria, angioedema, vomiting, abdominal pain, wheezing, cough, a feeling of suffocation, dizziness or anaphylaxis. The diagnosis starts with an allergy-focused clinical history, followed by tests looking for IgE sensitization, such as serum specific IgE or a skin prick test.
Insect venom allergy
An allergy consultation should be requested if, after a bee or wasp sting, generalised symptoms appear rather than a local reaction alone: widespread urticaria, swelling of the face or lips, difficulty breathing, a drop in blood pressure, fainting or anaphylaxis.
Contact dermatitis
Allergy is suspected when localised or recurrent eczema appears after contact with metals, fragrances, cosmetics, dyes, adhesives, gloves, occupational products or other substances applied to the skin.
This type of reaction is often delayed and is not investigated with specific IgE, but with patch testing.
When a reaction becomes a medical emergency
Signs that require calling 112 immediately
Difficulty breathing, swelling of the tongue or throat, sudden hoarseness, fainting, confusion or a reaction that worsens rapidly may indicate anaphylaxis. Call 112 immediately.