| Histamine (via H1–H4 histaminergic receptors) | Pruritus, tachycardia, rhinorrhoea, bronchospasm (H1) Endothelial release of nitric oxide (NO) leading to vasodilatation and hypotension (H1) Hypotension, flushing and headache (H1, 2) Inhibitory presynaptic (H3)—release endogenous epinephrine Chemotaxis and mast cell cytokine release (H4)
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| Tryptase | Activates: complement, coagulation and kalikrein–kinin system leading to angioedema, hypotension, and disseminated intravascular coagulation Respiratory and gastrointestinal tract mast cells contain less tryptase than connective tissue mast cells (tryptase may not increase in food anaphylaxis)
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| Platelet activating factor (PAF) | Responsible for systemic mast cell activation; high concentration of PAF and low concentrations of PAF-acetylhydrolase may predispose to severe reaction Low serum ACE concentrations may also contribute to severe anaphylaxis
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| Eosinophils | |