Residency · Residency · Allergy Immunology

IgE-Mediated Hypersensitivity and Mast Cell Biology

Gell and Coombs Classification Review

The classification of hypersensitivity reactions proposed by Gell and Coombs remains a foundational framework in clinical immunology, despite the recognition that many allergic and immune-mediated reactions involve overlapping mechanisms. | Type | Name | Mechanism | Timing | Clinical Examples |

Type I (immediate or IgE-mediated) hypersensitivity reactions occur within minutes of antigen exposure and are mediated by IgE-dependent mast cell and basophil degranulation. Type II (cytotoxic or antibody-mediated) reactions involve IgG or IgM antibodies directed against cell surface or matrix antigens, leading to complement-mediated cytolysis or antibody-dependent cellular cytotoxicity. Type III (immune complex) reactions result from the deposition of IgG or IgM immune complexes in tissues, activating complement and recruiting neutrophils. Type IV (delayed or T cell-mediated) reactions develop over hours to days and are mediated by effector T cells, with Pichler's subdivision into IVa through IVd further delineating specific effector mechanisms. This lecture focuses on Type I hypersensitivity, while recognizing that many clinical allergic reactions involve contributions from multiple hypersensitivity types operating in concert.

IgE Biology

Structure and Properties

Immunoglobulin E is the least abundant immunoglobulin isotype in serum, yet it wields extraordinary biological potency through its unique receptor-binding properties. The IgE molecule has a molecular weight of approximately 190 kDa and is composed of epsilon heavy chains, each containing four constant domains (CE1 through CE4) without a hinge region. The absence of a hinge region results in a relatively rigid molecular structure compared to IgG. The serum half-life of free IgE is remarkably short, approximately 2 days, making it the shortest-lived of all immunoglobulin isotypes. However, once bound to the high-affinity FcepsilonRI on tissue-resident mast cells, IgE persists for weeks to months, creating a state of long-lived sensitization. Serum IgE concentrations are the lowest of all immunoglobulin classes, typically 0.05 to 0.3 mcg/mL, representing only approximately 0.001 percent of total serum immunoglobulin. Normal total IgE in adults is less than 100 to 150 IU/mL, where 1 IU corresponds to 2.4 ng. Unlike IgG, IgE does not cross the placenta and does not activate complement.

IgE Class Switching

The generation of IgE requires two concurrent signals for B cell class switch recombination to the epsilon heavy chain locus. Signal 1 is provided by IL-4 or IL-13 engaging the IL-4 receptor alpha chain (IL-4Ralpha) on B cells, which activates STAT6 signaling and induces germline epsilon transcript transcription, opening the chromatin at the switch region upstream of the CE gene and making it accessible for AID-mediated recombination. Signal 2 is provided by CD40 ligand (CD40L/CD154) on T cells engaging CD40 on the B cell surface, which activates AID and the DNA recombination machinery necessary for class switch recombination. The dependence on both signals explains why IgE production requires cognate T cell help and why conditions that impair T cell function (such as CD40L deficiency) prevent IgE class switching.

An important observation is that IgE class switching frequently occurs through a sequential pathway, with B cells first switching from IgM to IgG4 (which shares IL-4 dependence for switching) and then from IgG4 to IgE, with deletion of the intervening switch regions. This sequential switch model has implications for understanding the relationship between IgG4 and IgE responses and the dynamics of immunotherapy.

IgE switching is negatively regulated by several cytokines, including IFN-gamma, IFN-alpha, TGF-beta, and IL-21, which suppress germline epsilon transcription or redirect class switching to other isotypes. Local IgE production within tissues, independent of systemic IgE levels, occurs in the nasal mucosa, bronchial tissue, and gut, and has clinical relevance in patients with normal total serum IgE who nevertheless demonstrate positive local IgE responses, a phenomenon best characterized in local allergic rhinitis.

IgE Receptors

Two distinct IgE receptors mediate the diverse biological effects of IgE. The high-affinity receptor FcepsilonRI binds IgE with extraordinary avidity (Kd approximately 10^-10 M), ensuring that IgE remains receptor-bound even at the vanishingly low concentrations present in serum. On mast cells and basophils, FcepsilonRI exists as a tetrameric complex (alpha-beta-gamma2), where the alpha chain provides the IgE-binding extracellular domain, the beta chain amplifies signaling through its ITAM, and the gamma chain homodimer provides the essential ITAMs for signal transduction. The alpha chain binds IgE at the CE3-CE4 junction of the epsilon heavy chain. On dendritic cells, monocytes, and eosinophils, FcepsilonRI exists as a trimeric form (alpha-gamma2) lacking the beta chain; this form mediates IgE-facilitated antigen capture and presentation rather than degranulation.

The low-affinity IgE receptor, FcepsilonRII (CD23), is a C-type lectin structurally unrelated to FcepsilonRI. It is expressed on B cells, monocytes, eosinophils, and epithelial cells. Membrane-bound CD23 provides negative feedback on IgE production by cross-linking with the BCR complex. Soluble CD23 (sCD23), generated by proteolytic cleavage, paradoxically enhances IgE production. CD23 also facilitates allergen transport across epithelial barriers, a function that may contribute to allergic sensitization through mucosal surfaces.

<image>A detailed molecular diagram of IgE binding to the high-affinity receptor FcepsilonRI on a mast cell surface. Show the IgE molecule with its four CE domains clearly labeled, with CE3 and CE4 making contact with the alpha chain of FcepsilonRI. The receptor is shown as tetrameric: alpha chain (extracellular Ig-like domains), beta chain (4 transmembrane passes with ITAM), and gamma chain homodimer (each with ITAM). Below the membrane, show the early signaling cascade: Lyn kinase associated with beta chain, phosphorylation of ITAMs, Syk kinase recruitment to gamma chain ITAMs, and downstream activation of PLCgamma, PI3K, and MAPK pathways. Adjacent panel shows allergen crosslinking two IgE-FcepsilonRI complexes to initiate degranulation.</image>

Mast Cell Biology

Mast Cell Development

Mast cells arise from CD34+ hematopoietic progenitors in the bone marrow but, unlike most other hematopoietic lineages, circulate as committed but immature progenitors (CD34+KIT+FcepsilonRI+) that complete their terminal differentiation only after arriving in peripheral tissues. This tissue-specific maturation is critically dependent on stem cell factor (SCF, also known as KIT ligand), which binds the receptor tyrosine kinase KIT (CD117) on mast cell progenitors and mature mast cells. KIT signaling is essential for mast cell development, survival, proliferation, and tissue homing. The activating KIT D816V mutation, which results in constitutive ligand-independent KIT signaling, is the molecular hallmark of systemic mastocytosis and is discussed in detail in lecture 28. Once established in tissues, mast cells are long-lived, persisting for months to years, and retain the capacity to proliferate locally, distinguishing them from terminally differentiated, short-lived granulocytes.

Mast Cell Heterogeneity

Human mast cells are classified into two principal subtypes based on their protease content. MCT (tryptase-only) mast cells are the predominant subtype in mucosal tissues, including the lung and intestinal mucosa, and their development is T cell-dependent. MCTC (tryptase and chymase) mast cells are the predominant subtype in connective tissues, including the skin and submucosa. The tissue microenvironment determines mast cell phenotype, and mast cells can change their protease expression profile when the local environment changes, demonstrating remarkable plasticity.

Mast Cell Mediators

Mast cell mediators are organized into three temporally distinct waves of release, each contributing different aspects of the allergic response.

Preformed mediators are stored in cytoplasmic granules and released within seconds of mast cell activation through compound exocytosis. Histamine, stored at 2 to 5 pg per mast cell, causes vasodilation, bronchoconstriction, and pruritus through H1 receptor activation on sensory nerves and vascular endothelium. Tryptase, a serine protease that serves as the most specific biomarker of mast cell activation, cleaves fibrinogen and activates PAR-2 (protease-activated receptor-2) on epithelial cells. Mature total tryptase (composed of alpha and beta forms) at baseline levels above 20 ng/mL raises suspicion for mastocytosis. In the context of suspected anaphylaxis, acute tryptase levels peak 60 to 90 minutes after symptom onset, and a value exceeding 1.2 times the baseline plus 2 ng/mL supports the diagnosis of mast cell activation. Chymase contributes to angiotensin II generation and tissue remodeling. Heparin, stored in complex with tryptase within granules, serves as an anticoagulant. TNF-alpha is unique among cytokines in being stored preformed in mast cell granules, enabling its immediate release. Carboxypeptidase A3 is another preformed granule protease.

Newly synthesized lipid mediators are generated within minutes of activation through the metabolism of membrane phospholipids. Prostaglandin D2 (PGD2) is the most abundant prostaglandin produced by mast cells and exerts its effects through two receptors: DP1 (causing vasodilation and, paradoxically, bronchodilation) and DP2/CRTH2 (mediating chemotaxis of Th2 cells, eosinophils, and basophils). Urinary PGD2 metabolites (9alpha,11beta-PGF2 and 2,3-dinor-11beta-PGF2alpha) serve as stable biomarkers for monitoring mast cell activation. The cysteinyl leukotrienes (LTC4, LTD4, LTE4), formerly known as slow-reacting substance of anaphylaxis (SRS-A), are approximately 1,000-fold more potent than histamine as bronchoconstrictors and also promote mucus secretion and vascular permeability. They act through CysLT1 receptors (blocked by montelukast and zafirlukast) and CysLT2 receptors. Urinary LTE4, a stable cysteinyl leukotriene metabolite, is characteristically elevated in aspirin-exacerbated respiratory disease. Platelet-activating factor (PAF) promotes platelet aggregation, bronchoconstriction, and hypotension.

De novo synthesized cytokines and chemokines are produced over hours through transcriptional activation and include IL-4, IL-5, IL-6, IL-13, IL-33, TNF-alpha, GM-CSF, and chemokines such as CCL2, CCL3, and CCL4. The production of IL-4 and IL-13 by mast cells can drive local IgE class switching in tissues, creating a self-perpetuating loop of IgE production and mast cell sensitization at sites of allergic inflammation.

<image>A three-panel timeline illustration of mast cell activation and mediator release. Panel 1 (Seconds - Preformed mediators): Mast cell with granule exocytosis releasing histamine, tryptase, heparin, TNF-alpha, chymase - with arrows pointing to their biological effects (vasodilation, bronchoconstriction, tissue degradation). Panel 2 (Minutes - Lipid mediators): Membrane phospholipid breakdown by PLA2, with arachidonic acid pathway branching to COX (generating PGD2) and 5-LO (generating LTC4/D4/E4), and PAF generation from lyso-PAF. Panel 3 (Hours - Cytokines/chemokines): Nuclear transcription and de novo synthesis of IL-4, IL-5, IL-13, TNF-alpha, and chemokines. Below all three panels, a graph showing the biphasic clinical response: early phase (0-60 min) and late phase (4-8 hours).</image>

Mechanism of IgE-Mediated Allergic Reaction

Sensitization Phase

The development of an IgE-mediated allergic response begins with the sensitization phase, which is clinically silent. Upon first exposure, allergen penetrates mucosal or cutaneous barriers and is captured by dendritic cells. Epithelial-derived alarmins (TSLP, IL-25, IL-33) promote dendritic cell maturation toward a Th2-promoting phenotype. In the draining lymph node, these dendritic cells present processed allergen to naive CD4+ T cells, polarizing them toward the Th2 lineage. Th2 cells produce IL-4 and IL-13 (providing Signal 1 for IgE class switching) and express CD40L (providing Signal 2), driving B cells to undergo class switch recombination to IgE. The resulting allergen-specific IgE is secreted by plasma cells, enters the circulation, and binds FcepsilonRI on tissue-resident mast cells, effectively "arming" them for future allergen encounter. This sensitization process occurs without any clinical symptoms.

Effector Phase (Early Phase Response)

Upon re-exposure, the multivalent allergen crosslinks two or more IgE-FcepsilonRI complexes on the mast cell surface, initiating a signaling cascade that begins with activation of the Src family kinase Lyn, which phosphorylates ITAMs on the beta and gamma chains of FcepsilonRI. The tyrosine kinase Syk is then recruited to the phosphorylated gamma chain ITAMs and activates downstream PLCgamma, PI3K, and MAPK pathways. Calcium influx from intracellular stores and the extracellular space triggers granule fusion with the plasma membrane through compound exocytosis. Clinical manifestations appear within minutes and include urticaria (dermal mast cell degranulation), rhinorrhea (nasal mucosal mast cells), bronchospasm (airway submucosal mast cells), and hypotension (systemic mediator release causing vasodilation and capillary leak). The early phase response typically resolves within 30 to 60 minutes.

Late Phase Response (LPR)

The late phase response occurs 4 to 8 hours after the initial allergen exposure and may persist for 24 to 48 hours. It is driven predominantly by the newly synthesized cytokines and lipid mediators released during the early phase, which recruit and activate a cellular infiltrate composed of eosinophils (recruited via IL-5 and eotaxin), basophils, Th2 cells, and neutrophils. Clinically, the late phase response is characterized by nasal congestion (rather than rhinorrhea, which predominates in the early phase), persistent bronchospasm, and induration at skin test sites. A critical clinical distinction is that the late phase response is steroid-responsive, whereas the early phase response is not, because corticosteroids suppress the transcription of cytokines and chemokines that drive the cellular infiltrate but do not prevent preformed mediator release.

Basophil Biology

Basophils are circulating granulocytes that constitute less than 1 percent of white blood cells and share several functional properties with mast cells, including expression of FcepsilonRI and production of histamine, LTC4, IL-4, and IL-13. However, important differences exist: basophils do not produce PGD2 (unlike mast cells), contain less histamine per cell (approximately 1 pg versus 2-5 pg in mast cells), and are terminally differentiated circulating cells rather than tissue-resident long-lived cells. The basophil activation test (BAT), which uses flow cytometry to measure upregulation of the activation markers CD63 or CD203c following allergen stimulation of whole blood, has emerged as a valuable research and clinical tool for evaluating drug allergy, food allergy, venom allergy, and for monitoring immunotherapy responses.

Histamine and Histamine Receptors

Histamine Receptor Subtypes

Histamine exerts its diverse biological effects through four receptor subtypes, each with distinct tissue distribution, signaling properties, and clinical applications. The H1 receptor is a Gq-coupled receptor that mediates smooth muscle contraction, vasodilation, pruritus, and increased vascular permeability, accounting for many of the cardinal symptoms of allergic reactions. H1 antihistamines (including second-generation agents such as cetirizine, loratadine, and fexofenadine) are competitive antagonists at this receptor. First-generation H1 antihistamines (diphenhydramine, chlorpheniramine) cross the blood-brain barrier and cause sedation and anticholinergic effects.

The H2 receptor is a Gs-coupled receptor that mediates gastric acid secretion and cardiac chronotropy while also exerting some suppressive effects on immune function. H2 antagonists (famotidine) provide additive benefit with H1 blockers in urticaria and anaphylaxis. The H3 receptor functions as a presynaptic autoreceptor in the central nervous system, modulating neurotransmitter release. The H4 receptor is expressed on eosinophils, mast cells, dendritic cells, and T cells, and mediates chemotaxis, representing a potential therapeutic target for allergic disease.

Anti-IgE Therapy

Omalizumab (Xolair)

Omalizumab is a humanized monoclonal antibody that has transformed the treatment of severe allergic disease. It binds to the CE3 domain of free IgE at the same epitope that engages FcepsilonRI, thereby preventing IgE from binding to its high-affinity receptor. Critically, omalizumab cannot bind IgE that is already bound to FcepsilonRI because the CE3 epitope is occupied by the receptor, which means omalizumab does not crosslink receptor-bound IgE and therefore does not trigger mast cell degranulation or anaphylaxis through this mechanism.

Omalizumab reduces free serum IgE by greater than 95 percent within days to weeks of initiating therapy. Over subsequent weeks to months, the loss of IgE occupancy of FcepsilonRI leads to progressive downregulation of receptor expression on mast cells and basophils, further reducing the sensitivity of these cells to allergen stimulation. This receptor downregulation accounts for the delayed full clinical benefit that typically requires 12 to 16 weeks to manifest.

Omalizumab is currently approved for moderate-to-severe allergic asthma (with dosing based on body weight and baseline total IgE), chronic spontaneous urticaria (with fixed dosing of 150 to 300 mg every 4 weeks regardless of IgE level or weight), chronic rhinosinusitis with nasal polyps, and IgE-mediated food allergy involving multiple foods. The anaphylaxis risk with omalizumab is approximately 0.1 to 0.2 percent, necessitating 30-minute observation for the first three injections, after which home administration may be considered.

An important practical consideration is that total serum IgE levels increase during omalizumab treatment because omalizumab-IgE complexes have a longer half-life than free IgE and are detected by standard IgE assays. Therefore, total IgE levels cannot be used to guide dosing adjustments during treatment.

<image>A mechanism-of-action illustration for omalizumab. Panel A: Without treatment - allergen crosslinks IgE on mast cell surface, triggering degranulation. Show high FcepsilonRI density. Panel B: Omalizumab binding - antibody shown binding to CE3 domain of free IgE molecule, preventing FcepsilonRI binding. Show omalizumab-IgE complex being cleared. Panel C: After weeks of treatment - mast cell with downregulated FcepsilonRI expression (fewer receptors on surface), reduced sensitivity to allergen. Include molecular detail of the CE3 binding epitope and why omalizumab cannot bind receptor-bound IgE (steric block). Label the time course: free IgE reduction within days, FcepsilonRI downregulation over weeks to months.</image>

Key Clinical Pearls

• Serum tryptase peaks 60-90 minutes after anaphylaxis onset and should be drawn during the event; compare to baseline (>=24 hours later)
• The late-phase response (4-8 hours) is steroid-responsive while the early phase is not - this is why steroids are given in anaphylaxis despite not treating the acute event
• Mast cells produce PGD2 but basophils do not - urinary PGD2 metabolites reflect mast cell activation specifically
• IgE class switching requires both IL-4/IL-13 (Signal 1) and CD40L-CD40 (Signal 2)
• Omalizumab binds free IgE at the same site as FcepsilonRI (CE3 domain), preventing receptor engagement without crosslinking receptor-bound IgE
• Total serum IgE levels rise during omalizumab treatment due to prolonged half-life of IgE-omalizumab complexes
• Baseline tryptase >20 ng/mL should raise concern for mastocytosis; hereditary alpha-tryptasemia (extra TPSAB1 copies) is a more common cause of mildly elevated baseline tryptase

References

1. Galli SJ, Tsai M. IgE and mast cells in allergic disease. Nat Med. 2012;18(5):693-704.
2. Kawakami T, Galli SJ. Regulation of mast-cell and basophil function and survival by IgE. Nat Rev Immunol. 2002;2(10):773-786.
3. Holgate ST, Djukanovic R, Casale T, Bousquet J. Anti-immunoglobulin E treatment with omalizumab in allergic diseases: an update on anti-inflammatory activity and clinical efficacy. Clin Exp Allergy. 2005;35(4):408-416.
4. Metcalfe DD, Peavy RD, Gilfillan AM. Mechanisms of mast cell signaling in anaphylaxis. J Allergy Clin Immunol. 2009;124(4):639-646.
5. Mukai K, Tsai M, Starkl P, Marichal T, Galli SJ. IgE and mast cells in host defense against parasites and venoms. Semin Immunopathol. 2016;38(5):581-603.

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