Immunology and AllergyTier 2Topic

Common allergic responses

Classification

Allergic (hypersensitivity) responses are classified into four types (Gell and Coombs), each with a distinct immune mechanism and clinical pattern
Atopic conditions (allergic rhinitis, asthma, eczema, food allergy) collectively affect a substantial proportion of the population and are rising in prevalence, particularly in industrialised settings

Gell and Coombs types

Type	Mechanism	Example
I - Immediate	IgE-mediated mast cell/basophil degranulation	Anaphylaxis, allergic rhinitis, urticaria
II - Antibody-mediated cytotoxic	IgG/IgM against cell-surface antigens	Haemolytic transfusion reaction, Goodpasture's
III - Immune complex	Antigen-antibody complexes deposit in tissue	Serum sickness, SLE
IV - Delayed (cell-mediated)	T cell-mediated, no antibody	Contact dermatitis, tuberculin reaction, drug rash

Timing and testing

Type I reactions occur within minutes; skin prick testing and specific IgE (RAST) identify the sensitising allergen
Type IV reactions take 24-72 hours to develop; patch testing identifies the causative contact allergen

Management

Type I - anaphylaxis: IM adrenaline first-line, antihistamines/corticosteroids as adjuncts only (not substitutes for adrenaline in anaphylaxis)
Allergen avoidance is central to management across all types once the trigger is identified
Immunotherapy (allergen desensitisation) - an option for Type I reactions to specific allergens (venom, some aeroallergens) with an established evidence base

Conditions

Anaphylaxis, allergic rhinitis, atopic dermatitis, contact dermatitis, drug allergy

The atopic march

The "atopic march" - allergic disease often progresses through a characteristic sequence in childhood (eczema -> food allergy -> allergic rhinitis -> asthma), though not universally or in a fixed order

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