Allergy and the Skin Flashcards
What is the function of the immune system?
Defence against external and internal antigens
What are some consequences of the immune system?
Intended destruction of antigens, incidental collateral damage of tissue
What is hypersensitivity?
Immune response that causes collateral damage to self due to exaggeration of normal immune mechanisms
What conditions is hypersensitivity the pathophysiological basis for?
Allergy, autoimmunity
Why do allergies occur?
A person’s immune system reacts to normally harmless substances in the environment
What are allergens?
A substance that causes an allergic reaction
What two factors interact to cause allergies?
Genetics and the environment
What are some factors that predispose to allergy?
Westernised countries, small family size, affluent urban homes, stable intestinal flora, high antibiotic use, low/absent helminth burden, good sanitation
What kind of reaction occurs in a type 1 allergy?
Immediate reaction within minutes up to 2hrs after exposure
What are some routes of exposure to allergens?
Skin contact, inhalation, ingestion, injection
What is the history of a patient with a type 1 allergy?
Consistent reaction with every exposure to allergen
What is the clinical presentation of an allergic reaction?
Urticaria, angioedema, wheezing/asthma, anaphylaxis
What are some features of urticaria?
Very itchy, lesions appear within 1hr, last 2-6 hrs, also called hives/wheals/nettle rash
What is angioedema?
Localised swelling of subcutaneous tissue/mucous membranes, non-pitting oedema, not itchy (unless associated with urticaria)
What is anaphylaxis?
Severe, life threatening generalised or systemic hypersensitivity reaction
What is anaphylaxis characterised by?
Rapidly developing life threatening problems with the airway (pharyngeal/laryngeal oedema), breathing (bronchospasm with tachypnoea) and circulation (hypotension and tachycardia)
When should serum mast cell tryptase levels be measured?
During anaphylaxis
What are some investigations that can be done for allergies?
Specific IgE (RAST), Challenge testing, Serum mast cell tryptase level, skin prick testing
What are the benefits of skin prick testing?
Cheap and quick, results in 15-20 mins, specificity and sensitivity of 90+%
Is there a risk of anaphylaxis with skin prick testing?
Yes- 1:3000
When should challenge testing be done?
Only if skin prick test is negative
What is the sensitivity and specificity of specific IgE (RAST) testing?
70-75%
How can allergies be managed?
Allergen avoidance, block mast cell activation and its effects, anti-inflammatory agents, adrenaline autoinjectors, immunotherapy, medic alert bracelet
What kind of drugs are used to prevent the effects of mast cell activation?
Anti-histamines
How many adrenaline autoinjectors should patients be prescribed?
2 pens
What are some anti-inflammatory agents used to treat allergies?
Corticosteroids
What condition are adrenaline autoinjectors used to prevent?
Anaphylaxis
How much adrenaline is pre-loaded in autoinjector pens?
300 micrograms for adults, 150 micrograms for children (new brand of injector has 500 micrograms for adults which is better dose)
What is an example of a drug which blocks mast cell activation?
Sodium cromoglicate-mast cell stabiliser
Are non-allergic reactions mediated by IgE?
No
What are some forms of non-allergic reaction?
Direct mass cell degranulation (morphine, aspirin, NSAIDS), metabolic (lactose intolerance), toxic (scombroid fish toxin), Type IV hypersensitivity
What are the features of type IV hypersensitivity?
Delayed hypersensitivity, antigen specific, T cell mediated, onset of reaction typically after 24-48hrs
What is an example of a skin condition caused by type IV hypersensitivity?
Allergic contact dermatitis
What are some routes of contact of substances that trigger non-allergic reactions?
Direct skin contact (preservatives within cosmetics), airborne contact (fragrance), injection (tattoos)
What does the presentation of a non-allergic reaction depend on?
Contact and exposure
What occurs in Patch testing?
Allergens prepared on Finn chambers, Finn chambers applied on the back and removed after 48hrs, readings taken at 48hrs and 96hrs
What is the sensitivity and specificity of patch testing?
70-80%
What is Irritant contact dermatitis?
Non-immunological process in which contact with agents abrades, irritates and traumatises the skin directly
Does Irritant contact dermatitis require previous sensitisation?
No
What are some examples of endogenous reactions?
Atopic eczema and psoriasis
Are endogenous reactions an allergic response?
No
What are some features of atopic asthma?
Dry skin and flexural, often associated with asthma and hayfever
What occurs in psoriasis?
Scaly plaques on extensor surfaces of skin
What are some ways to manage endogenous reaction and irritant contact dermatitis?
Irritant avoidance and minimisation, emollients, topical steroids, UV phototherapy, immunosuppressants
