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

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

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
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