Imm - Allergy Flashcards

1
Q

Define sensitisation

A

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

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2
Q

Describe the cellular response in allergy

A
  1. IL-25 and TSLP release from stressed/damaged epithelium
  2. Activation of Th2 cells
  3. Secretion of IL-4
  4. IL-4 stimulates B-cells to produce IgE and IgG4
  5. Cross-linking of IgE on mast cells → histamine, prostaglandins and leukotrienes
  6. Cytokines act on the endothelium → increased permeability, smooth muscle contraction
  7. IgE acts on eosinophils and basophils
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3
Q

What is the difference between oral and skin/resp exposure to allergens

A

Oral exposure → promotes immune tolerance (Due to T-regs from GI mucosa inhibiting IgE synthesis)
Skin and respiratory exposure → induces IgE sensitisation

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4
Q

What are the common allergies in infants vs children vs adults

A

Infants: atopic dermatitis, food (milk, egg, nits)
Children: allergic rhinitis, asthma
Adults: Drugs, bee stings, oral allergy syndrome, occupational

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5
Q

What constitutes sensitisation testing

A

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

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6
Q

What are the indications for IgE RAST testing

A

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

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7
Q

What are functional allergen tests

A

In vitro: basophil activation (measuring response to IgE-cross linking), serial mast cell tryptase
In vivo: open or blinded allergen challenge (gold standard)

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8
Q

Describe serial mast cell tryptase testing

A

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)

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9
Q

Describe the allergen challenge

A

Open or blinded
Gold standard for diagnosis of allergy
Increasing volumes of offending substance is ingest under close medical sueprvision

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10
Q

What are the mechanisms of anaphylaxis

A

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

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11
Q

What are the differentials for anaphylaxis

A

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

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12
Q

How is anaphylaxis diagnosed

A

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

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13
Q

What is the mechanism of IM adrenaline

A

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

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14
Q

Give examples of types of food intolerances

A

Food poisoning (bacterial, scromboid toxin)
Enzyme deficiencies (lactase)
Pharmacological (caffeine, tyramine)

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15
Q

Give examples of types of food allergy

A

IgE mediated reactions (anaphylaxis, OAS)
Mixed IgE and cell mediated (atopic dermatitis)
Non IgE mediated (coeliac disease)
Cell mediated (contact dermatitis)

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