Allergies: Mechanisms and causes
What is an allergy?
An allergy is an excessive or abnormal adaptive immune response directed against non-infectious, often inert environmental substances (allergens), including non-infectious components of certain infectious organisms.1
Allergic disorders, which include anaphylaxis, allergic rhinitis/hay fever, eczema, and asthma, afflict approximately 25% of people in the developed world. For people with allergies, persistent or repetitive exposure to allergens, which are typically innocuous substances, results in chronic allergic inflammation.1
In turn, this can produce long-term changes in the structure of the affected organs and substantial abnormalities in their function.1
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Triggers
Allergy triggers
There are two main types of allergen:1
1. Any non-infectious environmental substance that can induce immunoglobulin E (IgE) production (thereby ‘sensitising’ the subject), so that later re-exposure induces an allergic reaction.1 These include:
1. Pollens
Grass and tree pollens
2. Animal dander
Animal dander from skin and fur
3. Certain foods
Peanuts, tree nuts, fish, shellfish, milk, and eggs
4. Latex
Latex
5. Various medicines
Various medicines
6. Insect venom
Insect venom
Allergy triggers
2.A non-infectious environmental substance that can induce an adaptive immune response linked to local inflammation, but one thought to occur independently of IgE.1 These include:
1. Poison ivy or nickel
Allergic contact dermatitis to poison ivy or nickel
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Allergic response
What happens when an allergen infiltrates the body?
The body’s immune response to allergens includes a series of early- and late-phase reactions - the allergic cascade. There are several allergic mediators involved in this cascade, not just histamines.2
Typically, the allergic cascade follows this pattern:2
- Sensitisation to an allergen
- Early-phase response upon re-exposure to an allergen
- Late-phase response
Allergic inflammation:1
This is the inflammation produced in sensitised subjects after exposure to a specific allergen(s). With persistent or repetitive exposure to allergens, chronic allergic inflammation develops, with associated tissue alterations.1
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