NEW Added to close a syllabus gap — immunology sits under basic sciences, and anaphylaxis under complications of anaesthesia.

Immunology & anaphylaxis

The four hypersensitivity types, the immunoglobulins, and the perioperative anaphylaxis drill.

Hypersensitivity — the four types

Type	Mechanism	Timing	Example
I — Immediate	IgE on mast cells and basophils → degranulation	Minutes	Anaphylaxis, asthma, hay fever
II — Cytotoxic	IgG/IgM against cell-surface antigen → complement and cell lysis	Hours	ABO transfusion reaction, rhesus disease, myasthenia gravis, HIT
III — Immune complex	Antigen–antibody complexes deposit → complement activation	Hours to days	Serum sickness, SLE, post-streptococcal glomerulonephritis
IV — Delayed, cell-mediated	T lymphocytes — no antibody involved	2–3 days	Contact dermatitis, TB skin test, graft rejection

Mnemonic: Allergic · Cytotoxic · Immune complex · Delayed — "ACID".

Ig	Key facts
IgG	Most abundant. The only one that crosses the placenta — gives neonatal passive immunity. Main secondary response antibody
IgA	Predominant in secretions — saliva, tears, breast milk, mucosa
IgM	Pentamer, largest. First antibody in a primary response. Includes anti-A and anti-B isohaemagglutinins
IgE	Binds mast cells and basophils — type I hypersensitivity and parasitic infection
IgD	B-cell surface receptor; function uncertain

1. Neuromuscular blocking drugs — suxamethonium and rocuronium most often.
2. Antibiotics — teicoplanin and co-amoxiclav prominent.
3. Chlorhexidine — frequently missed; think of it with delayed onset and central line or urinary catheter insertion.
Then latex, colloids, patent blue dye, and protamine.

Under anaesthesia the first signs are often cardiovascular: unexplained hypotension and tachycardia, high airway pressures and desaturation, with the rash unnoticed under drapes.
Stop the trigger. Call for help. 100% oxygen. Secure the airway.
Adrenaline — 500 µg IM (0.5 ml of 1:1000) if no IV access; 50 µg IV boluses titrated where access exists. Infusion if repeated boluses are needed.
Aggressive IV fluid — large volumes are often required as capillary leak is profound.
Second line: chlorphenamine 10 mg and hydrocortisone 200 mg. Salbutamol for persistent bronchospasm. Consider glucagon if on beta blockers.

Mast cell tryptase — sample as soon as feasible, at 1–2 hours, and a baseline at 24 hours or in clinic. Tryptase peaks within about an hour and has a short half-life.
Refer to an allergy clinic for skin prick and intradermal testing at 4–6 weeks. Document clearly and give the patient written information.
Anaphylactoid reactions are clinically identical but non-IgE mediated — direct mast cell degranulation, no prior exposure needed (atracurium, morphine, vancomycin "red man"). Management is the same.