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PH-IM01

Anaphylaxis

How IgE primes mast cells, why re-exposure to an allergen releases histamine and other mediators throughout the body, and how leaky vessels and swollen airways cause shock and obstruction within minutes.

By the end, you can

  • Describe the normal roles of mast cells, IgE and the vascular endothelium.
  • Explain how sensitisation primes mast cells and basophils with allergen-specific IgE.
  • Explain how cross-linking of IgE triggers degranulation, and name the main mediators released.
  • Relate vasodilation and capillary leak to distributive shock, urticaria and angioedema.
  • Relate laryngeal oedema and bronchospasm to stridor and wheeze.
  • Recognise the diagnostic criteria, grading, tests and mimics of anaphylaxis, and explain how adrenaline’s receptor actions and the other treatments act on its mechanism.

What it is

Anaphylaxis is a severe, generalised hypersensitivity reaction that develops rapidly and threatens the airway, breathing or circulation, usually with skin and mucosal changes. Most cases are IgE-mediated (type I hypersensitivity); in others, mast cells are activated by routes that do not need IgE.

2 types to tell apart
IgE-mediated (allergic)
Needs earlier sensitisation: the allergen cross-links IgE already bound to mast cells and basophils. Foods, insect venoms and drugs such as penicillins are typical triggers.
Non-IgE-mediated
Mast cells are activated directly or through complement, for example by some contrast media or opioids. No sensitisation is needed, so it can happen on a first exposure; the mediators and effects are the same.

Normal function

Mast cells, IgE and healthy vessels

live in connective tissue throughout the body, most densely round and under the surfaces that meet the outside world: the skin, the airways and the gut. Their cytoplasm is packed with granules of ready-made chemical messengers (mediators), chiefly histamine and enzymes such as tryptase. Basophils, white cells of the blood, carry similar granules.

Both cells have high-affinity receptors (FcεRI) for the tail of IgE, an antibody class that make in tiny amounts. IgE normally helps defend against parasites, and a local release of mast-cell granules is useful: it widens nearby vessels and makes them leaky, so plasma proteins and white cells reach a site of injury or infection.

Elsewhere, the tone of the arterioles holds blood pressure up, and tight junctions between endothelial cells keep most plasma protein inside the vessels, so fluid stays in the circulation. Smooth muscle sets the width of the bronchi and bronchioles.

Point to it on the model