Allergy & The Skin Flashcards
What mediates type 1 hypersensitivity/allergy
IgE
When does a type 1 hypersensitivity/allergy present with respect to allergen exposure
Immediate reaction - occurs within minutes & up to 2 hours after exposure to allergen
What route of exposure causes type 1 hypersensitivity/allergy
Skin contact e.g. latex
Inhalation e.g. pollen, house dust mite
Ingestion e.g. nuts, seafood
Injection e.g. bee sting, medications
Describe the pathophysiology of type 1 allergy
- Sensation stage: production of specific IgE by B cells (helped by T cells) in response to initial allergen exposure, residual IgE antibodies bind to circulating mast cells via Fc receptors
- Allergic stage: on re-exposure to allergen, the allergen will bind to IgE coated mast cells → cell degranulation (release of histamine and other inflammatory mediators)
Type 1 allergy clinical features
- Urticaria (wheels & hives)
- Angioedema (localised, non pitting oedema)
- GI symptoms (N&V, abdominal pain, diarrhoea)
- Respiratory (wheezing, sneezing, nasal itching, rhinorrhoea)
- Anaphylaxis
Oedema in allergy vs swelling secondary to atopic dermatitis - how do you tell the difference
Atopic dermatitis is likely to be scaly, itchy, weeping etc with less pronounced swelling
Allergic oedema is likely to be accompanied by hives & other allergic symptoms
Type 1 Allergy investigations
- History
- Specific IgE (RAST) blood test
- Skin prick test (if IgE negative)
- Challenge test (only if SPT is negative)
Anaphylaxis investigation
Serum mast cell tryptase level (during anaphylaxis)
Type 1 allergy management
- Allergy avoidance
- Anti histamines (prevents effects of mast cells) (1st line)
- Corticosteroids (anti inflammatory agent) (2nd line)
- Adrenaline auto injector (for anaphylaxis)
- Sodium cromoglicate (mast cell stabiliser/blocker)
When does a type IV hypersensitivity/allergy present with respect to allergen exposure
Delayed hypersensitivity - Onset of reaction typically after 12-24 hours
What mediates Type IV allergy
T Cell mediated, antigen specific
What is type IV allergy aka
Allergic contact dermatitis
What route of exposure causes type IV hypersensitivity/allergy
Direct skin contact e.g. cosmetic preservative, nickel in belt
Airborne contact e.g. fragrances, plants
Injection e.g. tattoo
Type IV Allergy investigations
Patch testing
What can be mistaken for allergic contact dermatitis
Irritant contact dermatitis
Endogenous dermatitis e.g. eczema
Infection
Irritant/ Allergic contact dermatitis management
•Allergen / Irritant avoidance
•Allergen / Irritant minimisation
•Emollients
•Topical steroids
•UV phototherapy
•Immunosuppressants
Type IV allergy/ allergic contact dermatitis pathophysiology
- Sensation stage: generation of memory T cells following exposure to antigen (via Langerhans cells in epidermis and MHC-II)
- Allergic stage: activation of sensitised Th cells in response to antigen, causing release of inflammatory cytokines and cell-mediated cytotoxicity
Allergic contact dermatitis clinical features
itchy, eczematous rash (vesicles, fissures, erythema)
Allergic vs irritant contact dermatitis clinical features
- In allergic contact dermatitis the rash may extend beyond the boundaries of immediate contact
- This is unlike irritant contact dermatitis, where the skin changes are localised directly to the area of exposure
- Allergic contact dermatitis requires a sensitisation phase whereas irritant contact dermatitis doesn’t
Common irritants
Detergents, bleach, PPD, nappy rash (urine - flexures will be spared), ‘lip-lick’ chelitis
Main skin differences in urticaria (true allergic reaction) vs dermatitis
Urticaria
- Smooth & well defined
Dermatitis
- Rough & ill defined
Acute food allergy (Type I, IgE mediated) 3 step management
- At first sign of reaction - chlorphenamine & If asthmatic & wheezing - inhaler
- Not resolving/ worsening after 30 mins - prednisolone & call for help
- If becomes blue/ collapses - IM adrenaline & call 999
