Which antibody drives immediate (type I) hypersensitivity?

IgE. Allergen-specific IgE binds high-affinity receptors on mast cells and basophils, and allergen cross-linking of this IgE triggers the rapid mediator release that characterises immediate hypersensitivity.

How do type II and type III hypersensitivity differ?

Type II reactions involve antibody binding antigens on a cell or tissue surface, whereas type III reactions involve soluble antigen-antibody complexes that deposit in tissues; both then recruit complement and inflammatory effectors.

Hypersensitivity Reactions and IgE-Mediated Immunity

Antibodies normally protect, but the same effector mechanisms can cause tissue-damaging reactions to otherwise harmless antigens. The Gell and Coombs scheme classifies these hypersensitivity reactions by mechanism, and the antibody-dependent types, especially IgE-driven immediate hypersensitivity, illustrate how isotype and effector pathway determine the kind of damage produced.

Definition

Hypersensitivity reactions are immune responses that cause tissue injury on contact with an antigen; in the antibody-mediated types this is driven by specific isotypes, with IgE mediating immediate (type I) reactions through sensitised mast cells and basophils and IgG mediating cytotoxic (type II) and immune-complex (type III) reactions.

Scope

The topic covers the Gell and Coombs classification of hypersensitivity (types I-IV), the central role of IgE in immediate hypersensitivity through mast cells and basophils, and the antibody-mediated type II and type III mechanisms, with attention to how IgG subclasses contribute. It is reference immunology that describes mechanisms and explicitly avoids diagnostic or treatment advice.

Core questions

How does the Gell and Coombs scheme classify hypersensitivity by mechanism?
Why is IgE central to immediate hypersensitivity?
How do mast cells and basophils translate IgE binding into a reaction?
How do antibody-mediated type II and type III reactions differ from IgE-driven ones?

Key concepts

Type I (immediate) hypersensitivity
Type II (cytotoxic) hypersensitivity
Type III (immune complex) hypersensitivity
Type IV (delayed-type) hypersensitivity
IgE and high-affinity FcεRI
Mast cell and basophil degranulation
Allergen sensitisation
IgG subclass contributions

Key theories

Gell and Coombs classification: Hypersensitivity is grouped into four mechanistic types: IgE-mediated immediate (I), antibody-mediated cytotoxic (II), immune-complex (III), and T cell-mediated delayed (IV), a framework that organises otherwise diverse reactions by their effector basis.

Mechanisms

In immediate (type I) hypersensitivity, an initial sensitisation generates allergen-specific IgE that binds the high-affinity receptor FcεRI on mast cells and basophils; on re-exposure the allergen cross-links this IgE, triggering degranulation and release of histamine and other mediators within minutes. Type II reactions are caused by IgG or IgM binding antigens on cell surfaces, leading to complement-mediated and Fc-receptor-mediated damage. Type III reactions arise when antigen-antibody (immune) complexes deposit in tissues and activate complement and inflammatory cells. Type IV reactions, included in the scheme for completeness, are mediated by T cells rather than antibody. IgG subclasses differ in their ability to fix complement and engage Fc receptors, which shapes the antibody-mediated types.

Clinical relevance

The classification underlies how allergic, cytotoxic, and immune-complex disorders are understood mechanistically and how allergy testing and antibody-directed therapies are conceived. The entry presents these mechanisms for reference and is not a guide to diagnosis or management; it gives no dosing or individualized advice.

History

Gell and Coombs introduced their four-type classification of hypersensitivity in the 1960s, giving a durable mechanistic vocabulary. The identification of IgE as the reaginic antibody of immediate hypersensitivity, and the later characterisation of its high-affinity receptor and of mast-cell and basophil biology, established the molecular basis of allergic reactions.

Key figures

Robin Coombs
Philip Gell
Dean Metcalfe
Kimishige Ishizaka

Seminal works

stone-2010
gell-coombs-1963

Frequently asked questions

Which antibody drives immediate (type I) hypersensitivity?: IgE. Allergen-specific IgE binds high-affinity receptors on mast cells and basophils, and allergen cross-linking of this IgE triggers the rapid mediator release that characterises immediate hypersensitivity.
How do type II and type III hypersensitivity differ?: Type II reactions involve antibody binding antigens on a cell or tissue surface, whereas type III reactions involve soluble antigen-antibody complexes that deposit in tissues; both then recruit complement and inflammatory effectors.