Type I Hypersensitivity (Immediate)

Type I (immediate) hypersensitivity is an IgE-mediated reaction in which previously sensitised mast cells and basophils release preformed and newly synthesised mediators within minutes of re-exposure to an antigen. It underlies common allergic disorders and, in its most severe form, anaphylaxis.

Definition

Type I hypersensitivity is an immediate, IgE-dependent reaction in which antigen cross-links IgE bound to high-affinity FcεRI receptors on mast cells and basophils, triggering degranulation and release of histamine, leukotrienes, prostaglandins, and cytokines that produce the allergic response.

Scope

This entry covers the sensitisation and effector phases of immediate hypersensitivity, the central role of IgE and mast cells, the early and late reaction phases, and the clinical spectrum from local allergy to systemic anaphylaxis. It is a mechanistic reference within immunopathology and is not allergy management guidance.

Core questions

• How does first exposure to an allergen produce IgE that arms mast cells?
• What happens at the molecular level when allergen cross-links cell-bound IgE?
• Why does an immediate reaction sometimes have a delayed late phase?
• What distinguishes localised allergy from systemic anaphylaxis?

Key concepts

• IgE and FcεRI
• Mast cell and basophil degranulation
• Th2 response and IL-4/IL-5/IL-13
• Sensitisation versus elicitation
• Histamine and lipid mediators (leukotrienes, prostaglandins)
• Early-phase and late-phase reaction
• Anaphylaxis

Mechanisms

In an initial sensitisation phase, allergen presented in a Th2-skewed context drives B cells to class-switch to IgE, which binds high-affinity FcεRI receptors on mast cells and basophils. On re-exposure, allergen cross-links adjacent IgE molecules, triggering receptor aggregation and cellular activation. This causes immediate degranulation with release of preformed histamine and proteases, followed by synthesis of lipid mediators such as leukotrienes and prostaglandins and of cytokines. The result is vasodilation, increased vascular permeability, smooth-muscle contraction, and mucus secretion within minutes (early phase), often followed hours later by an eosinophil-rich late phase. When mediator release is systemic, the result is anaphylaxis with hypotension and airway compromise.

Clinical relevance

Immediate hypersensitivity is the mechanism behind allergic rhinitis, allergic asthma, urticaria, food and drug allergy, and anaphylaxis, and it explains why these reactions occur rapidly on re-exposure. The entry describes the mechanism and its clinical correlates for orientation; it is not a guide to diagnosis or treatment of allergic disease.

Epidemiology

IgE-mediated allergic conditions are among the most prevalent immune disorders, affecting a substantial proportion of the population in many regions, with rising prevalence reported over recent decades. Anaphylaxis is comparatively uncommon but potentially life-threatening; precise rates depend on trigger and setting.

Evidence & guidelines

Consensus definitions of anaphylaxis and its recognition have been articulated in expert symposium reports; mechanistic understanding rests on immunology reviews. Disease-specific management is addressed in the relevant clinical entries and external guidelines.

History

The phenomenon of anaphylaxis was described in the early twentieth century, but the immunological basis of immediate hypersensitivity became clear with the identification of IgE as the responsible antibody class in the 1960s, after which the mast cell-IgE axis was defined in molecular detail.

Key figures

• Kimishige Ishizaka
• Teruko Ishizaka
• Dean Metcalfe

Seminal works

• stone-2010
• sampson-2006

Frequently asked questions

Why is Type I hypersensitivity called immediate?

Because in a sensitised person the reaction begins within minutes of antigen exposure, driven by preformed IgE on mast cells that releases mediators almost instantly upon allergen cross-linking.

What is the late-phase reaction?

After the immediate response subsides, an eosinophil- and cytokine-driven inflammatory phase can develop several hours later at the same site, contributing to sustained symptoms in conditions such as asthma.