Type 1 hypersensitivity reactin mechanism

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Type 1 hypersensitivity reaction mechanism IgE mast cells

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Type I Hypersensitivity Reaction - Mechanism

Type I hypersensitivity (immediate hypersensitivity) is an IgE-mediated allergic reaction that occurs within minutes of re-exposure to an antigen (allergen). It involves two key phases: sensitization and the effector (elicitation) phase.

Sequence of events in immediate hypersensitivity reactions

Phase 1: Sensitization (First Exposure)

This phase primes the immune system - no symptoms occur yet.

Allergen entry: Allergens (pollen, house dust mite proteins, food proteins, drugs) enter via mucosal surfaces and are captured by dendritic cells (DCs).
T cell polarization: DCs present allergen peptides to naive T cells, which differentiate into Th2 cells (and T follicular helper [Tfh] cells). Th2/Tfh cells produce IL-4 and IL-13.
IgE class switching: IL-4 and IL-13 act on allergen-specific B cells, promoting class-switch recombination from IgM/IgG to IgE production. Plasma cells then secrete large amounts of IgE.
Mast cell sensitization: IgE binds with extremely high affinity to FcεRI (Fc epsilon receptor type I) on tissue mast cells and circulating basophils. The mast cells are now "armed" - this is sensitization.

At this point the person is sensitized but asymptomatic.

Phase 2: Effector Phase (Re-exposure / Elicitation)

Re-exposure to the same allergen triggers the reaction.

Allergen cross-linking: The allergen binds to and cross-links two or more adjacent IgE molecules on the mast cell surface. This aggregates the FcεRI receptors.
Mast cell activation: Cross-linking of FcεRI activates intracellular signaling (Lyn kinase → Syk kinase → downstream phospholipase C pathway), leading to:
- Rise in intracellular Ca²+
- Degranulation - preformed granule contents released within seconds to minutes

Mediators Released

Category	Mediator	Effect
Preformed (granules)	Histamine	Vasodilation, increased vascular permeability, smooth muscle contraction
	Heparin	Anticoagulant
	Tryptase, chymase	Tissue remodeling, inflammation marker
	Serotonin	Vasoconstriction/dilation depending on site
Newly synthesized lipid mediators	Prostaglandin D2 (PGD2)	Vasodilation, bronchoconstriction
	Leukotrienes C4, D4, E4	Prolonged bronchoconstriction, mucus secretion, increased vascular permeability
	PAF (platelet-activating factor)	Vasodilation, leukocyte chemotaxis
Cytokines (produced on activation)	IL-4, IL-13	Sustain Th2 responses, mucus secretion
	IL-5	Eosinophil recruitment and activation
	TNF, IL-3, MIP-1α	Late-phase inflammation

Cellular and Molecular Immunology, Table 20.2

Immediate vs. Late-Phase Reactions

Immediate reaction (within minutes): Driven by vasoactive amines (histamine) and lipid mediators - causes rapid vasodilation, bronchoconstriction, and mucus secretion.
Late-phase reaction (2-4 hours after re-exposure): Driven by cytokines (especially IL-4, IL-5, TNF) released by mast cells. These recruit eosinophils, neutrophils, and more Th2 cells, causing a prolonged inflammatory response.

Clinical Manifestations

Depending on the tissue involved:

Systemic - Anaphylaxis (most severe - airway restriction + cardiovascular collapse)
Respiratory - Allergic asthma, allergic rhinitis (hay fever)
Skin - Urticaria (hives), angioedema, atopic dermatitis
GI - Food allergy symptoms (abdominal pain, diarrhea)
Eyes - Allergic conjunctivitis

Key diagnostic markers:

Serum tryptase - elevated in anaphylaxis (peaks at ~1 hour; also chronically elevated in systemic mastocytosis)
Plasma histamine - peaks rapidly (~10 minutes), returns to normal within 1 hour
Urinary histamine - may remain elevated for 24 hours

Katzung's Basic and Clinical Pharmacology, 16th Ed., pp. 1524-1525

Cellular and Molecular Immunology (Abbas), 10th Ed., pp. 1302-1308

Quick Compendium of Clinical Pathology, 5th Ed., p. 4360

Why NOT Type I: Important Distinctions

Anaphylactoid reactions - clinically similar to anaphylaxis but NOT IgE-mediated (direct mast cell degranulation by drugs like morphine, radiocontrast media) - not true Type I hypersensitivity
Hereditary angioedema - complement-mediated (C1 inhibitor deficiency) - not Type I
Type II - antibody-mediated cytotoxicity (IgG/IgM on cell surfaces)
Type III - immune complex-mediated (IgG + antigen complexes + complement)
Type IV - delayed-type (T cell and macrophage mediated, 48-72 hrs)

Haptens

Haptens are small molecules that can bind antibodies but cannot trigger an immune response on their own - they need to couple with a larger carrier protein to become immunogenic. Classic examples include penicillin, poison ivy (urushiol), and DNCB.

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hapten immunology mechanism carrier protein immune response

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Haptens

Definition

A hapten is a small chemical molecule (typically <1 kDa) that is antigenic (can bind to a specific antibody) but not immunogenic by itself (cannot elicit an immune response alone). To become immunogenic, a hapten must be covalently coupled to a large carrier protein - this complex is called the hapten-carrier conjugate or immunogen.

The term comes from the Greek haptein ("to fasten"). The concept was first worked out using small molecules like m-aminobenzene sulfonate and dinitrophenol (DNP).

"Immunization with free hapten produces no antibodies to the hapten. However, immunization with hapten groups linked to a protein carrier generates antibodies that react with high affinity to hapten alone or linked to a molecule other than the carrier."

Roitt's Essential Immunology

The Classic Demonstration

Antigenicity vs Immunogenicity - hapten-carrier experiment

As shown above:

Injecting a free hapten (e.g., m-aminobenzene sulfonate) alone → no antibody produced
Injecting hapten conjugated to a carrier protein (e.g., ovalbumin) → produces anti-hapten AND anti-carrier antibodies
The antibodies produced can then react with the free hapten in vitro

Why Haptens Alone Cannot Induce an Immune Response

The reason lies in the T cell-B cell collaboration required for a full humoral immune response:

B cell antigen presentation and T cell recognition

A T cell-dependent B cell response requires two distinct epitopes on the same molecule:

Role	Recognized by	Epitope type
Hapten	B cell (via BCR/membrane Ig)	Native conformational epitope
Carrier protein	CD4+ T helper cell (via class II MHC)	Linear peptide epitope

A hapten alone is too small to be processed into peptides and presented on MHC class II - so it cannot activate T helper cells. Without T cell help, B cells cannot undergo full activation, class switching, affinity maturation, or plasma cell differentiation.

Three Key Rules of the Hapten-Carrier Effect (Abbas, Cellular & Molecular Immunology)

Both hapten-specific B cells AND carrier-specific T helper cells are required - the response is T cell-dependent
Hapten and carrier must be physically linked - administering them separately cannot induce an anti-hapten response. The hapten is responsible for the efficient internalization of the carrier protein into the B cell
The interaction is MHC class II-restricted - helper T cells cooperate only with B cells that express the same class II MHC molecules involved in the initial T cell activation by dendritic cells

Mechanism Step-by-Step

Hapten-carrier conjugate enters the body
Hapten-specific B cell binds the conjugate via its BCR (recognizing the hapten epitope)
The B cell endocytoses the entire hapten-carrier conjugate
The carrier protein is proteolytically processed inside the B cell into linear peptides
These peptides are loaded onto class II MHC molecules and displayed on the B cell surface
A carrier-specific CD4+ T cell (Th cell) recognizes the class II MHC-peptide complex
T cell delivers help via CD40L-CD40 interaction + cytokines (IL-4, IL-21)
B cell activates, proliferates, undergoes isotype switching, and differentiates into plasma cells secreting anti-hapten antibodies

Clinical Relevance

Drug allergies are the most important clinical application of hapten biology:

Drug	Mechanism
Penicillin	The reactive β-lactam ring covalently binds to carrier proteins, forming a hapten. This is why ~10% of penicillin-allergic patients also react to cephalosporins (cross-reactive haptens)
Cephalosporins	Similar β-lactam haptenization
Tetracyclines, Tolbutamide	Hapten/drug adsorption mechanism

Drug-induced immune hemolytic anemia uses the hapten mechanism:

Drugs like penicillin bind to RBC membranes (haptenize them)
Anti-drug IgG antibodies then coat the RBCs → complement activation → hemolysis (Type II hypersensitivity)

Application: Conjugate Vaccines

The hapten-carrier concept is the basis for conjugate vaccines against encapsulated bacteria:

Polysaccharide capsule antigens (equivalent to haptens - recognized by B cells but T cell-independent and weak in infants) are covalently linked to carrier proteins (e.g., tetanus toxoid, CRM197)
This converts the weak T-independent polysaccharide response into a strong T-dependent response with isotype switching, affinity maturation, and immunologic memory
Examples: Hib (H. influenzae type b), PCV (pneumococcal), MCV (meningococcal) vaccines

Cellular and Molecular Immunology (Abbas), 10th Ed., pp. 769-771

Roitt's Essential Immunology, p. 163

The Washington Manual of Medical Therapeutics, p. 1215

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