Imm - Allergy Flashcards
Define sensitisation
The detection of specific IgE either by skin prick testing or in vitro blood tests
Demonstrates risk of allergic disorder but does not define an allergic disorder
Describe the cellular response in allergy
- IL-25 and TSLP release from stressed/damaged epithelium
- Activation of Th2 cells
- Secretion of IL-4
- IL-4 stimulates B-cells to produce IgE and IgG4
- Cross-linking of IgE on mast cells → histamine, prostaglandins and leukotrienes
- Cytokines act on the endothelium → increased permeability, smooth muscle contraction
- IgE acts on eosinophils and basophils
What is the difference between oral and skin/resp exposure to allergens
Oral exposure → promotes immune tolerance (Due to T-regs from GI mucosa inhibiting IgE synthesis)
Skin and respiratory exposure → induces IgE sensitisation
What are the common allergies in infants vs children vs adults
Infants: atopic dermatitis, food (milk, egg, nits)
Children: allergic rhinitis, asthma
Adults: Drugs, bee stings, oral allergy syndrome, occupational
What constitutes sensitisation testing
Allergen-specific IgE: skin prick or IgE RAST
IgE: positive control (histamine) and negative control dilutent)
- Positive = wheal >3mm than negative control
- Discontinue antihistamines 48h beforehand
IgE RAST (radioallergosorbent) blood tests
- Pt serum added to the allergen → detection of IgE via anti-IgE Abs tagged with a label
Note: tests for sensitisation, NOT clinical allergy
What are the indications for IgE RAST testing
Patients who can’t stop antihistamines (otherwise do skin test)
Patients with dermatographism
Patients with extensive eczema
History of anaphylaxis
Borderline/equivocal skin prick test results
What are functional allergen tests
In vitro: basophil activation (measuring response to IgE-cross linking), serial mast cell tryptase
In vivo: open or blinded allergen challenge (gold standard)
Describe serial mast cell tryptase testing
Tryptase = pre-formed protein in mast cell granules → released in anaphylaxis
Peak concentration = 1-2 hours; baseline = 6-12 hours
Useful if diagnosis of anaphylaxis uncertain (e.g. hypotension/rash in anaesthesia)
Describe the allergen challenge
Open or blinded
Gold standard for diagnosis of allergy
Increasing volumes of offending substance is ingest under close medical sueprvision
What are the mechanisms of anaphylaxis
IgE: mast cells and basophils via histamine and platelet activating factor (PAF)
IgG: macrophages and neutrophils via histamine and PAF
Complement: mast cells and macrophages via histamine and PAF
Pharm: mast cells via leukotrienes and histamine
What are the differentials for anaphylaxis
C1 inhibitor deficiency (C1 esterase)
Chronic urticaria and angioedema
MI and PE
Very severe asthma, inhaled foreign body
Anxiety or panic disorder
Carcinoid and phaeochromocytoma
How is anaphylaxis diagnosed
Clinical
If unsure”
Serial measurement of serum tryptase (1, 3, 24h)
The rise in tryptase concentration is directly proportional to fall in BP
DIAGNOSIS = persistent rise in tryptase 24 hours after allergic reaction suggestive of systemic mast cell disease
What is the mechanism of IM adrenaline
Alpha 1 – causes peripheral vasoconstriction, reverses low BP and mucosal oedema
Beta 1 – increases heart rate, contractility and BP
Beta 2 – relaxes bronchial smooth muscle and reduces the release of inflammatory mediators
Give examples of types of food intolerances
Food poisoning (bacterial, scromboid toxin)
Enzyme deficiencies (lactase)
Pharmacological (caffeine, tyramine)
Give examples of types of food allergy
IgE mediated reactions (anaphylaxis, OAS)
Mixed IgE and cell mediated (atopic dermatitis)
Non IgE mediated (coeliac disease)
Cell mediated (contact dermatitis)
