Allergies: Mechanisms and causes

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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:

    Woman sniffing flowers

    1. Pollens

    Grass and tree pollens

    Children playing with a puppy

    2. Animal dander

    Animal dander from skin and fur

    Two women sitting by a lake

    3. Certain foods

    Peanuts, tree nuts, fish, shellfish, milk, and eggs

    Family and dog on a camping trip

    4. Latex


    Clinician smiling

    5. Various medicines

    Various medicines

    Family in a lakeside garden

    6. Insect venom

    Insect venom

    Allergy triggers

    There are two main types of allergen:1

    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:

    Family walking in the woods

    1. Poison ivy or nickel

    Allergic contact dermatitis to poison ivy or nickel

  • Allergic response

    Woman with red, watering eyes

    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

    1. Sensitisation to an allergen
    2. Early-phase response upon re-exposure to an allergen
    3. 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

Read more about allergies

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