Non-clinical sciences/Immunology · Hypersensitivity

Type I hypersensitivity

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What it means

Immediate, IgE-mediated. Allergen cross-links IgE on mast cells → degranulation within minutes → histamine/leukotrienes → vasodilation, bronchospasm, oedema. Spectrum: anaphylaxis, allergic rhinitis/asthma, urticaria/angio-oedema, atopic eczema.

Worked example

Anaphylaxis: IM adrenaline 500 µg (0.5 mL 1:1000) anterolateral thigh, repeat at 5 min; the rest is supportive (Resus Council UK 2021).

In the exam

Within minutes of a wasp sting a man develops urticaria, wheeze, angio-oedema and hypotension.

What settles it

Type I is minutes & IgE/mast cell; type II is antibody vs a cell-surface antigen; type III is immune-complex; type IV is delayed T-cell (48–72 h).

Classically confused with

Type IV hypersensitivity

Source: StatPearls; Resuscitation Council UK 2021