Allergies: Causes and mechanisms

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:

1. Pollens

Grass and tree pollens

2. Pets

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

3. Certain foods

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

4. Latex

5. Various medicines

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

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:

1. Poison ivy or nickel

Allergic contact dermatitis to poison ivy or nickel
Allergic response
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

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

Product Information - Pirinase Allergy

Pirinase Allergy 0.05% Nasal Spray (fluticasone propionate). Uses: Prevention and treatment of allergic rhinitis. Dosage and method of use: Adults: Two sprays into each nostril once or twice a day if required. 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 18 years: Not to be used. Contraindications: Hypersensitivity to ingredients. Precautions: Seek medical advice if: improvement is not seen within 7 days, relief is inadequate after 7 days, or more than 3 months continuous use is required. Seek medical advice before use with: other corticosteroid products, fever, nasal/sinus infection, 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, particularly if impairment of adrenal function suspected. Systemic effects of nasal corticosteroids may occur, particularly at high doses prescribed for prolonged periods. May cause visual disturbance. 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 £7.24; 120 sprays £11.66. Date of last revision: April 2023.
Product information – Piriton Syrup and Piriton Allergy Tablets

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: 4 mg 4-6 hourly. Max 24 mg/24h. Elderly: Lower dose should be considered. Children 6-12 years: 2 mg 4-6 hourly. Max 12 mg/24h. Piriton Allergy Tablets not recommended for children under 6 years. Do not use either product continuously for more than two weeks without consulting a doctor. Syrup only: Children 2-6 years: 2.5 ml (1 mg) 4-6 hourly. Max 6 mg/24h. Children 1-2 years: 2.5 ml (1 mg), twice daily. Max 2 mg/24h. Not recommended for children below 1 year. Contraindications: Hypersensitivity. 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 enlarged prostate, severe hypertension, cardiovascular disease or respiratory conditions, hepatic or renal impairment, epilepsy, raised intraocular pressure, bronchitis, bronchiectasis and asthma. 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, hydroxybenzoates (E218, E214, E216) which may cause allergic reactions, and ethanol (6.3% v/v). Side effects: Sedation, somnolence, fatigue, disturbance in attention, abnormal coordination, dizziness, headache, blurred vision, nausea, dry mouth, allergic disorders including anaphylaxis. Children and the elderly are more prone to the neurological anticholinergic effects and rarely may become confused or excitable. 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 £5.42. Tablets: 30s £4.58, 60s £7.92. Date of last revision: January 2024.
Product Information – Piriteze

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 daily. Children 6-12 years: 5 mg twice daily. Elderly: As adults if renal function normal. Allergy Tablets and Syrup: Children under 6 years: Not recommended. P-line Syrup only: Children 2 to 6 years: 2.5mg twice daily. Children under 2 years: Not recommended. 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. Tablets: Not suitable for those with galactose intolerance, Lapp lactase deficiency or glucose-galactose malabsorption. Syrup: Not to be taken by those with fructose intolerance. Contains methyl and propyl parahydroxybenzoate which may cause allergic reactions. Side effects: See SPC for full details. Dizziness, headache, fatigue, somnolence, dry mouth, sore throat, abdominal pain, nausea, diarrhoea, rhinitis, allergic disorders including anaphylaxis and hypersensitivity, suicidal ideation, depression, nightmares, inflammation of the liver, skin eruptions, joint pain. Legal category: GSL: Syrup (6years+) and Tablets: 70ml/ 7, 14, 30 P: Syrup (2years+): 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 £3.73, 14s £6.66, 30s £9.16. Syrup (GSL): 70 ml £5.41. Syrup (P): 70ml £5.41 Date of last revision: Aug 2023.
Product Information - Pirinase Hayfever

Pirinase Hayfever Once Daily 0.05% Nasal Spray (fluticasone propionate). Indications: Treatment of 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: Seek medical advice if inadequate relief after 7 days or require more than 1-month continuous use. Seek medical advice before use with: corticosteroid use, 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. Systemic effects of nasal corticosteroids may occur, particularly at high doses prescribed for prolonged periods. May cause visual disturbance. Contains benzalkonium chloride which may cause bronchospasm. Side effects: Dryness and irritation of the nose and throat, nasal ulcers, nasal septal perforation, unpleasant taste and smell, headache and epistaxis. Hypersensitivity reactions including skin rash and oedema of the face or tongue. Eye disorders such as glaucoma, raised intraocular pressure, cataract and blurred vision. Anaphylaxis/ anaphylactic reactions and bronchospasm. 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 £7.92. Text prepared: April 2023.