Allergy is a type I hypersensitivity reaction in which the immune system mounts an IgE-mediated inflammatory response against innocuous environmental antigens (allergens). This represents a failure of immune tolerance and misdirected Th2 immunity, where Mast cells and basophils degranulate upon allergen re-exposure, releasing vasoactive mediators that cause immediate symptoms. Allergic diseases reflect evolutionary mismatch between modern immune programming and ancestral pathogen diversity.
Think of your immune system as a border security force. Normally, it distinguishes dangerous smugglers (pathogens) from harmless tourists (pollen, peanuts, cat dander). In allergy, the security force mistakes tourists for terrorists—a catastrophic intelligence failure.
The first time a tourist (allergen) crosses the border, undercover agents (B cells) secretly take their photo and mass-produce "WANTED" posters (IgE antibodies). These posters are plastered all over alarm bells throughout the city (FcεRI receptors on mast cells). The tourist walks through town unmolested—no symptoms.
But when that same tourist returns, every alarm bell recognizes their face. Two adjacent WANTED posters get cross-linked by the tourist's presence, triggering a city-wide emergency broadcast: mast cells explode open, releasing sirens (Histamine), emergency flares (leukotrienes), and inflammatory chemicals. Blood vessels dilate (flushing, swelling), airways constrict (wheezing), and mucus floods the scene. Within minutes, the entire city shuts down—all because of a harmless visitor. The system is sensitized to overreact, and it cannot forget.
- Allergen Presentation: Allergen (e.g., pollen protein Bet v 1, peanut Ara h 2) crosses mucosal barriers → captured by dendritic cells
- Th2 Polarization: Dendritic cells migrate to lymph nodes → present allergen peptides via MHC class II to naïve CD4+ T cells
- Cytokine Milieu: In the absence of strong Th1 signals (low IL-12, low IFN-γ), dendritic cells produce IL-4 and thymic stromal lymphopoietin (TSLP) → drives differentiation into Th2 cells
- B Cell Class Switching: Th2 cells release IL-4, IL-5, IL-13
- IL-4 binds B cell IL-4 receptor → activates STAT6 → upregulates activation-induced cytidine deaminase (AID)
- AID catalyzes class switch recombination: IgM/IgD → IgE
- IgE Arming: IgE antibodies circulate and bind with picomolar affinity to FcεRI (high-affinity IgE receptor) on Mast cells, basophils, eosinophils
- Mast cells now "armed" with allergen-specific IgE—sensitization complete (asymptomatic)
- Allergen Cross-linking: Upon re-exposure, allergen simultaneously binds ≥2 adjacent IgE molecules on mast cell surface → FcεRI cross-linking
- Mast Cell Degranulation (occurs within seconds to minutes):
- FcεRI cross-linking → activates Lyn and Syk kinases → phosphorylate LAT and SLP-76 → activate phospholipase C-γ (PLCγ)
- PLCγ cleaves PIP₂ → IP₃ + DAG
- IP₃ → Ca²⁺ release from endoplasmic reticulum → degranulation
- Preformed Mediator Release:
- Histamine: binds H1 receptors (vascular endothelium, bronchial smooth muscle) → vasodilation, bronchoconstriction, pruritus
- Tryptase: serum tryptase >11.4 ng/mL diagnostic for anaphylaxis; degrades neuropeptides
- Heparin: anticoagulant, amplifies vascular permeability
- Newly Synthesized Mediators (minutes to hours):
- Late-Phase Reaction (4-12 hours post-exposure):
- IL-5, IL-13, eotaxin recruit eosinophils
- Eosinophils release major basic protein, eosinophil peroxidase → tissue damage
- Sustained inflammation, airway remodeling in chronic disease
graph TD
A[Allergen crosses mucosa] --> B[Dendritic cell uptake]
B --> C[MHC-II presentation to naive T cell]
C --> D{Cytokine environment}
D -->|IL-4, TSLP| E[Th2 differentiation]
D -->|"IL-12, IFN-γ"| F[Th1 - tolerance]
E --> G[B cell activation]
G -->|"IL-4 → STAT6 → AID"| H[Class switch to IgE]
H --> I["IgE binds FcεRI on mast cell"]
I --> J[Sensitization complete]
K[Re-exposure to allergen] --> L[IgE cross-linking on mast cell]
L --> M["Lyn/Syk → PLCγ → Ca²⁺"]
M --> N["IMMEDIATE: Degranulation"]
N --> O[Histamine, tryptase, heparin]
M --> P["MINUTES: Lipid mediators"]
P --> Q[Leukotrienes LTC4/D4/E4]
P --> R[Prostaglandins PGD2/E2]
N --> S["HOURS: Cytokines IL-5, IL-13"]
S --> T[Eosinophil recruitment]
T --> U[Late-phase inflammation]
Systemic allergen exposure → massive mast cell degranulation across multiple organs → circulatory collapse:
- Widespread vasodilation → hypotension (systolic BP <90 mmHg)
- Plasma extravasation → hypovolemic shock
- Laryngeal/bronchial edema → airway obstruction
- Epinephrine (adrenaline) 0.3-0.5 mg IM reverses via β₂-adrenergic bronchodilation and α₁ vasoconstriction
¶ Prevalence and Evolutionary Context
Allergic diseases affect 20-30% of Western populations but remain rare in traditional hunter-gatherer societies. This reflects the hygiene hypothesis and old friends mechanism: reduced childhood exposure to helminth parasites, bacteria, and viruses fails to educate Regulatory T cells (Tregs) and skews toward Th2 dominance. The immune system evolved expecting early-life pathogen exposure to calibrate tolerance—modern hygiene breaks this contract.
¶ Conditions and Patient Profiles
- Asthma: Chronic airway inflammation, reversible bronchoconstriction; IgE-mediated in extrinsic asthma
- Allergic rhinitis: "Hay fever"; IgE to pollen, dust mites → nasal congestion, rhinorrhea
- Atopic dermatitis (eczema): IgE + barrier dysfunction (Filaggrin mutations)
- Food allergy: IgE to foods (peanut, shellfish, milk); oral tolerance failure
- Anaphylaxis: Medical emergency; tryptase >11.4 ng/mL confirms mast cell activation
- Atopic march: Sequential development—eczema (infancy) → food allergy → rhinitis → asthma
- Metamodel 0 (Evolution): Allergies are diseases of evolutionary mismatch—immune systems programmed for pathogen-rich environments malfunction in sterile modernity
- Selfish Immune System: Th2 immunity diverts resources to fight "false threats," draining energy from repair (Th1/Treg balance)
- Leaky gut: Intestinal hyperpermeability allows food proteins to cross intact → sensitization (zonulin, dysbiosis)
- Chronic stress: Cortisol initially suppresses Th1 → compensatory Th2 dominance; chronic stress → cortisol resistance → unchecked inflammation
¶ Biomarkers and Thresholds
- Total IgE: >100 IU/mL suggests atopy (normal <50 IU/mL)
- Specific IgE: >0.35 kU/L to particular allergen (e.g., Ara h 2 for peanut)
- Eosinophils: >500 cells/μL in peripheral blood
- Serum tryptase: Baseline >20 ng/mL → systemic mastocytosis; acute >11.4 ng/mL → anaphylaxis
- Exhaled nitric oxide (FeNO): >50 ppb in allergic asthma
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Restore Immune Tolerance:
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Address Barrier Dysfunction:
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Modulate Th2 Pathways:
- Anti-IgE monoclonal antibody (omalizumab) for severe asthma
- Leukotriene receptor antagonists (montelukast)
- Quercetin, Luteolin: natural mast cell stabilizers
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Address Dysbiosis:
-
Reduce Chronic Stress:
-
Acute Management:
- Antihistamines (H1 antagonists)
- Inhaled β₂-agonists (albuterol) for asthma
- Epinephrine auto-injector for anaphylaxis risk
- Type I hypersensitivity: IgE-mediated, mast cell-dependent, immediate-onset (minutes)
- Sensitization phase asymptomatic: First exposure generates IgE but no symptoms
- FcεRI affinity: 10⁻¹⁰ M (picomolar)—highest affinity receptor-ligand in immunology
- Immediate reaction: 5-30 minutes (histamine, tryptase)
- Late-phase reaction: 4-12 hours (eosinophils, tissue damage)
- Anaphylaxis criteria: Acute onset with skin/mucosal + respiratory or cardiovascular compromise
- Tryptase half-life: 90 minutes—measure within 1-3 hours of suspected anaphylaxis
- Cysteinyl leukotrienes: 100-1000× more potent bronchoconstrictors than histamine
- Atopic march sequence: Eczema → food allergy → rhinitis → asthma
- Western prevalence: 20-30% vs. <5% in traditional societies (e.g., hunter-gatherers, rural Africa)
- Epinephrine dose: 0.3-0.5 mg IM (adults), repeat every 5-15 min if needed
- IgE — The central antibody class in allergy; high-affinity binding to FcεRI arms mast cells
- Th2 immunity — Th2 polarization drives IgE class switching and eosinophil recruitment; allergies represent inappropriate Th2 dominance
- Mast cells — Effector cells that degranulate in response to allergen-IgE cross-linking, releasing histamine and lipid mediators
- Histamine — Immediate vasodilator and bronchoconstrictor released from mast cell granules; primary target of antihistamines
- Leukotrienes — Lipid mediators synthesized de novo via 5-LOX; cysteinyl leukotrienes cause potent bronchoconstriction in asthma
- Eosinophils — Granulocytes recruited in late-phase allergic reactions; release toxic granule proteins (major basic protein)
- Regulatory T cells — Treg dysfunction or deficiency allows unchecked Th2 responses; helminth exposure promotes Treg induction
- Hygiene hypothesis — Reduced pathogen exposure in early life fails to educate immune tolerance, increasing allergy risk
- Dysbiosis — Gut microbial imbalance reduces SCFA production and Treg induction, promoting Th2 skewing
- Leaky gut — Increased intestinal permeability allows food proteins to cross intact, facilitating sensitization
- IL-4 — Master Th2-polarizing cytokine; drives IgE class switching via STAT6
- IL-5 — Recruits and activates eosinophils in late-phase reactions
- IL-13 — Promotes IgE production, mucus hypersecretion, airway remodeling in asthma
- TSLP — Epithelial alarmin that drives dendritic cells toward Th2-promoting phenotype
- Chronic stress — Chronic cortisol exposure → cortisol resistance → Th2 skew and increased allergy susceptibility
- Vitamin D — VDR signaling promotes Treg function and modulates Th2 responses; deficiency associated with atopy
- Short-chain fatty acids — Butyrate and propionate promote Treg differentiation via GPR43/GPR109A; reduced in dysbiosis
- Filaggrin — Skin barrier protein; loss-of-function mutations increase atopic dermatitis and food sensitization risk
- Inflammasome — NLRP3 activation can promote Th2 responses; chronic activation in allergy
- Asthma — Chronic inflammatory airway disease; IgE-mediated bronchial mast cell degranulation in extrinsic subtype
- Atopic dermatitis — Skin barrier dysfunction + IgE-mediated inflammation; first step in atopic march