Allergies: Causes and mechanisms

Allergy lifestyle image

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

44% of British adults suffer from at least one allergy and this is on the rise, and 48% of sufferers have more than one allergy according to data from 2010.2 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:

    Allergy pollen lifestyle

    1. Pollens

    Grass and tree pollens

    Allergy animal lifestyle

    2. Pets

    Animal dander from skin and fur, pet salivia and pet urine

    Allergy food lifestyle

    3. Certain foods

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

    Allergy latex lifestyle

    4. Latex

    Allergy Various medicines lifestyle

    5. Various medicines

    Common triggers of drug allergies include penicillin and related antibiotics, anticonvulsants, aspirin, ibuprofen and other NSAIDs and chemotherapy drugs.3

    Allergy insect venom lifestyle

    6. Insect venom

    Allergies to bee and wasp stings or insect bites. Less common in children.

    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:

    Allergy poison Ivy or nickel lifestyle

    1. Poison ivy or nickel

    Allergic contact dermatitis to poison ivy or nickel

  • Allergic response

    Allergy reaction lifestyle

    What happens when an allergen infiltrates the body?

    In allergic individuals 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.4

    Typically, the allergic cascade follows this pattern:4

    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

  • Product Information - Pirinase Allergy

    Full information can be found in the Summary of Product Characteristics.

    Pirinase Allergy 0.05% w/w Nasal Spray (fluticasone propionate). Uses: Prevention and treatment of allergic rhinitis. Dosage and method of use: Adults, elderly and adolescents aged 12 years and over: Two sprays into each nostril once a day, preferably morning. Twice a day may be required in some cases. Maintenance: one spray per nostril once a day if possible. Use lowest possible maintenance dose. Max 4 sprays a day in each nostril. Children under 12 years: Not to be used. Contraindications: Hypersensitivity to ingredients. Concomitant use with HIV medicines. Precautions: Stop treatment or seek medical advice if improvement is not seen within 7 days. Seek medical advice if relief is inadequate, or if more than 3 months continuous use is required. Abnormally heavy challenge of summer allergens may in certain instances necessitate appropriate additional therapy. Seek medical advice before use with: other corticosteroid products, fever, nasal/sinus infection (treat appropriately), recent nasal injury/surgery, nasal ulceration. Caution due to interactions with cytochrome P450 3A4 inhibitors may increase systemic exposure to fluticasone. Caution commencing in those withdrawing from systemic steroids, and commencing therapy with intranasal fluticasone propionate, particularly if impairment of adrenal function suspected. Treatment with higher than recommended doses of nasal corticosteroids may result in clinically significant adrenal suppression. If there is evidence of higher than recommended doses being used, then additional systemic corticosteroid cover should be considered during periods of stress or elective surgery. Systemic effects of nasal corticosteroids may occur, particularly at high doses prescribed for prolonged periods. These may include Cushing's syndrome, Cushingoid features, adrenal suppression, growth retardation in children and adolescents, cataract, glaucoma and more rarely, a range of psychological or behavioural effects including psychomotor hyperactivity, sleep disorders, anxiety, depression or aggression (particularly in children). May cause visual disturbance. Blurred vision or other visual disturbances should be considered for referral to an ophthalmologist for evaluation. Contains benzalkonium chloride which may cause bronchospasm. Side effects: Very Common: epistaxis. Common: Dryness and irritation of the nose and throat, unpleasant taste and smell, headache. See SPC for full details. Legal category: P. PL number: PL 44673/0099. Product licence holder: Haleon UK Trading Limited, The Heights, Weybridge, Surrey, KT13 0NY, UK. Package quantity and MRSP (excl. VAT): 60 sprays £8.32; 120 sprays £12.37. Date of last revision: Feb 2026.

  • Product information – Piriton Syrup and Piriton Allergy Tablets

    Full information can be found in the Summary of Product Characteristics.

    Piriton Children’s Allergy Syrup and Piriton Allergy Tablets (chlorphenamine maleate) Indications: Symptomatic relief of allergic conditions including hay fever. Symptomatic relief of itch associated with chickenpox. Dosage and method of use: Adults and children 12 years and over: 10ml or 1 tablet (4mg) 4-6 hourly. Max 60ml or 6 tablets (24mg) in any 24 hours. Elderly: Lower dose should be considered. Children 6-12 years: 5 ml or 1/2 tablet (2mg) 4-6 hourly. Max 30ml or 3 tablets (12mg) in any 24 hours. Syrup only: Children 2-6 years: 2.5 ml (1 mg) 4-6 hourly. Max 15ml (6 mg) in any 24 hours. Children 1-2 years: 2.5 ml (1 mg), twice daily. Max 5ml (2 mg) in any 24 hours. Piriton Allergy Tablets not recommended for children under 6 years, and Piriton Syrup not recommended for children below 1 year. Do not use either product continuously for more than two weeks without consulting a doctor. Contraindications: Hypersensitivity to antihistamines or the product ingredients. Treatment with MAOIs within last 14 days. Warnings and Precautions: Avoid concurrent use with other antihistamines and alcohol. May seriously affect ability to drive and use machinery. Caution in patients with: enlarged prostate, severe hypertension or cardiovascular disease, hepatic or renal impairment, epilepsy, raised intraocular pressure, bronchitis, bronchiectasis and asthma. Children and elderly more likely to experience neurological anticholinergic effects and paradoxical excitation. Avoid use in elderly patients with confusion. Avoid concurrent use with drugs which cause sedation. Those with fructose or galactose intolerance, Lapp lactase deficiency, glucose-galactose malabsorption or sucrose-isomaltase insufficiency should not take these medicines. Syrup: Contains sucrose (should be considered in those with diabetes mellitus, ensure adequate dental hygiene is maintained), hydroxybenzoates (E218, E214, E216) which may cause allergic reactions (possibly delayed), and ethanol (6.3% v/v) (should be considered in those suffering from alcoholism and other high-risk groups). Side effects: Very common: Sedation, somnolence. Common: Fatigue, disturbance in attention, abnormal coordination, dizziness, headache, blurred vision, nausea, dry mouth. See SPC for full details. Legal category: P. PL numbers: Syrup: PL 44673/0094. Tablets: PL 44673/0093. Product licence holder: Haleon UK Trading Limited, The Heights, Weybridge, Surrey, KT13 0NY, UK. Package quantity and RSP (excl. VAT): Syrup: 150 ml £6.04. Tablets: 30s £5.42, 60s £8.75. Date of last revision: Jan 2026

  • Product Information – Piriteze

    Full information can be found in the Summary of Product Characteristics.

    Piriteze Hayfever & Allergy 10mg Film Coated Tablets, Piriteze Children's Hayfever & Allergy 1mg/ml Syrup (GSL)and Piriteze Children's Hayfever & Allergy 1mg/ml Syrup (P) (cetirizine hydrochloride). Indication: Symptomatic treatment of seasonal and perennial allergic rhinitis, and chronic idiopathic urticaria. Dosage and method of use: Adults and adolescents over 12 years: 10 mg once daily. Children 6-12 years: 5 mg twice daily. Elderly: As adults if renal function normal. Syrups only: Children 2 to 6 years: 2.5mg twice daily. Tablets not recommended for children under 6 years and syrups not recommended for children under 2 years. Adjust dose in moderate to severe renal impairment. See SPC for details. Contraindications: Hypersensitivity to ingredients, hydroxyzine, piperazine derivatives or severe renal impairment (creatinine clearance <10 ml/min). Warnings and Precautions: Concomitant alcohol. Predisposition factors of urinary retention (e.g. spinal cord lesion, prostatic hyperplasia). Epilepsy/risk of convulsions. Pruritus and/or urticaria may occur when cetirizine stopped, may require treatment to be restarted. Allergy skin tests are inhibited by antihistamines, a wash out period (3 days) is required before performing them. Tablets: Not suitable for those with galactose intolerance, Lapp lactase deficiency or glucosegalactose malabsorption. Syrup: Not to be taken by those with fructose intolerance. Contains methyl and propyl parahydroxybenzoate which may cause allergic reactions (possibly delayed). Side effects: Uncommon: Agitation, paraesthesia, diarrhoea, pruritus, rash, asthenia, malaise. All other side effects are rare, very rare or have an unknown frequency see SPC for full details. Legal category: GSL: Syrup and Tablets: 70ml/ 7, 14, 30 P: Syrup: 70ml. PL numbers: Tablets: PL 44673/0097. Syrup (GSL): PL 44673/0095. Syrup (P): PL 44673/0096. Product licence holder: Haleon UK Trading Limited, The Heights, Weybridge, Surrey, KT13 0NY, United Kingdom Package quantity and RSP (excl. VAT): Tablets: 7s £4.16, 14s £6.66, 30s £9.16. Syrup (GSL): 70 ml £5.41. Syrup (P): 70ml £5.41 Date of last revision: January 2026

  • Product Information - Pirinase Hayfever

    Full information can be found in the Summary of Product Characteristics.

    Pirinase Hayfever Once Daily 0.05% Nasal Spray (fluticasone propionate). Indications: Treatment of seasonal allergic rhinitis. Dosage and method of use: Adults: Two sprays into each nostril once a day, preferably in the morning. Maintenance: one spray per nostril once a day, if symptoms recur the dosage may be increased accordingly. Use lowest possible maintenance dose. Max 2 sprays a day in each nostril. Children under 18 years: Not to be used. Contraindications: Hypersensitivity to ingredients. Concomitant use with HIV medicines. Warnings & Precautions: Stop treatment or seek advice if an improvement is not seen within 7 days. Seek medical advice if relief is inadequate after 7 days or more than 1-month continuous use is required. Seek medical advice before use with: other corticosteroids, nasal/sinus infection, recent nasal injury/surgery, nasal ulceration. Caution due to interactions with potent cytochrome P450 3A4 inhibitors which may increase systemic exposure to fluticasone. Treatment with higher than recommended doses of nasal corticosteroids may result in clinically significant adrenal suppression. If there is evidence of higher than recommended doses being used, then additional systemic corticosteroid cover should be considered during periods of stress or elective surgery. Systemic effects of nasal corticosteroids may occur, particularly at high doses prescribed for prolonged periods. May cause visual disturbance. Blurred vision or other visual disturbances should be considered for referral to an ophthalmologist for evaluation. Contains benzalkonium chloride which may cause bronchospasm. Side effects: See SPC for full details. Very Common: Epistaxis. Common: Headache, unpleasant taste and smell, dryness and irritation of the nose and throat. Legal category: GSL. Product licence number: PL 44673/0100. MAH: Haleon UK Trading Limited, The Heights, Weybridge, Surrey, KT13 0NY, UK. RRP (excl. VAT): 60 sprays £8.32. Text prepared: Jan 2026.

Oral antihistamines and Intranasal steroids (INS) – first line recommendations for allergic rhinitis5

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