Allergy and Hypersensitivity

Why allergy and hypersensitivity matter

• It is a defense system turned against the self. The same IgE, mast cells, and eosinophils that evolved to expel worms and venoms now misfire against pollen, peanuts, penicillin, and cat dander. Hypersensitivity is the immune system's precision turned into a liability — the mechanisms are exquisite; the target is wrong.
• Allergy is common and rising. Roughly 10–30% of people in industrialized countries have allergic rhinitis (hay fever), around 8% of children carry a food allergy, and asthma affects more than 260 million people worldwide. Prevalence has climbed steeply over the past half-century, tracking the industrialized lifestyle.
• Anaphylaxis can kill in minutes. A systemic Type I reaction drops blood pressure, swells the airway, and constricts the bronchi so fast that death can occur within 5–30 minutes of exposure. Peanut, tree-nut, shellfish, insect venom, and drugs like penicillin are leading triggers, which is why epinephrine auto-injectors exist.
• One framework explains transfusion reactions, lupus, and poison ivy at once. The Gell–Coombs scheme unifies phenomena that look unrelated in the clinic — a mismatched blood transfusion (Type II), the joint pain of serum sickness (Type III), and a nickel rash from a watch (Type IV) are all hypersensitivity, just via different effectors.
• It underpins modern immunotherapy. Allergen desensitization (subcutaneous or sublingual immunotherapy) retrains the response over years, and biologic drugs targeting the pathway — omalizumab against IgE, dupilumab against the IL-4 receptor alpha shared by IL-4 and IL-13, mepolizumab against IL-5 — now treat severe asthma, chronic urticaria, and eczema by dismantling the Type I machinery at defined nodes.
• It is a natural experiment in immune tolerance. Why the immune system tolerates food proteins in most people but not in the allergic minority is one of immunology's central questions; oral tolerance, regulatory T cells, and the timing of first exposure (the LEAP trial showed early peanut introduction cuts allergy) all bear on it.

Common misconceptions

• "Allergy and hypersensitivity are the same thing." Allergy is only Type I — the IgE-mediated, immediate, histamine-driven subset. Hypersensitivity is the umbrella covering all four Gell–Coombs types. A poison-ivy rash is colloquially an "allergy," but mechanistically it is Type IV (T-cell) hypersensitivity, with no IgE and no histamine spike.
• "The first exposure causes the reaction." The first exposure is silent — it only sensitizes, installing allergen-specific IgE on mast cells. Symptoms need a second (or later) encounter that cross-links that pre-loaded IgE. A first bee sting or first penicillin dose is usually uneventful; the danger comes later.
• "An allergic reaction means the allergen is dangerous." The allergen itself is usually harmless — pollen, peanut protein, a cat's Fel d 1. The damage comes entirely from the host's own overreaction. Allergy is a failure of tolerance, not a property of the antigen.
• "Antihistamines will stop anaphylaxis." Antihistamines block only the H1 (and H2) actions of histamine, and only slowly. Anaphylaxis is driven by many mediators (leukotrienes, prostaglandins, tryptase, PAF) across multiple organs. Only epinephrine reverses the airway edema, bronchospasm, and shock in time. Antihistamines and steroids are adjuncts, never a substitute.
• "Histamine is stored and that's all mast cells do." Preformed histamine, tryptase, and heparin are released in seconds, but mast cells then synthesize a second wave — leukotrienes C4/D4/E4 and prostaglandin D2 within minutes, and cytokines (TNF, IL-5, IL-13) over hours. The late-phase reaction 4–8 hours later, driven by these and by recruited eosinophils, is why symptoms can rebound after the acute event settles.
• "A positive skin test means you'll have a reaction." Skin-prick and specific-IgE tests detect sensitization (the presence of IgE), not clinical allergy. Many sensitized people tolerate the food or allergen fine. Diagnosis requires correlating the test with a real-world reaction, and the gold standard remains a supervised oral food challenge.

How allergy and hypersensitivity work

The defining feature of Type I allergy is that it is a two-phase story: sensitization, then re-exposure. During sensitization, a dendritic cell samples an allergen at a barrier surface — the airway, gut, or skin — and presents it to a naive CD4 T cell. In the allergic response, that T cell polarizes toward the Th2 lineage and secretes interleukin-4 and interleukin-13. These cytokines drive allergen-specific B cells to class-switch their heavy chain to the ε constant region, producing IgE. The secreted IgE then diffuses through tissue and binds the high-affinity receptor FcεRI on mast cells and basophils with a dissociation constant near 10⁻¹⁰ M — so tight that mast cells stay armed for weeks to months. At the end of sensitization the person is primed but symptom-free.

The effector phase begins on the next encounter. A multivalent allergen bridges two adjacent IgE molecules on the mast cell surface, cross-linking their FcεRI receptors. Receptor aggregation activates the Src-family kinase Lyn and then Syk, which trigger phospholipase C-γ, generate IP₃ and diacylglycerol, and drive a rapid rise in cytosolic calcium. Within seconds the mast cell degranulates, exocytosing preformed granules loaded with histamine, tryptase, chymase, and heparin. Histamine acting on H1 receptors causes vasodilation, increased vascular permeability (the wheal and flare), itch, and bronchial and gut smooth-muscle contraction. Over the next minutes the cell synthesizes lipid mediators — cysteinyl leukotrienes (C4, D4, E4) and prostaglandin D2 — and over hours releases cytokines (TNF, IL-5, IL-13) that recruit eosinophils and drive the late-phase reaction. When this cascade fires locally you get hives, rhinitis, or an asthma attack; when it fires systemically you get anaphylaxis: airway edema, bronchospasm, and distributive shock.

The other three types use different effectors. Type II hypersensitivity is IgG or IgM binding an antigen fixed on a cell surface or in the extracellular matrix — the antibody then triggers complement-mediated lysis, opsonization and phagocytosis, or antibody-dependent cellular cytotoxicity (as in ABO transfusion reactions, Rh hemolytic disease of the newborn, and autoimmune hemolytic anemia). Type III hypersensitivity is soluble antigen–antibody immune complexes that fail to be cleared, deposit in vessel walls, glomeruli, and joints, and activate complement to recruit neutrophils — the mechanism of serum sickness, the local Arthus reaction, and the vasculitis of lupus. Type IV hypersensitivity is delayed and T-cell-mediated: a hapten (nickel ion, urushiol, a drug) modifies self-protein, is presented on MHC to memory Th1 cells, which secrete IFN-γ and recruit macrophages 48–72 hours later, producing the induration of a positive tuberculin test or the plaques of allergic contact dermatitis. Type IV needs cells, not serum — it cannot be transferred by antibody, only by T cells.

The four Gell–Coombs types compared

Feature	Type I (immediate)	Type II (cytotoxic)	Type III (immune complex)	Type IV (delayed)
Mediator	IgE	IgG / IgM (on cell surface)	IgG / IgM (soluble complexes)	T cells (Th1, CTL)
Key effector cell	Mast cell, basophil	NK cell, phagocyte, complement	Neutrophil, complement	Macrophage, cytotoxic T cell
Timing	Minutes	Minutes to hours	Hours (~6 h)	Delayed, 48–72 h
Chief molecule	Histamine, leukotrienes	Complement (MAC), FcγR	C3a/C5a, deposited C3b	IFN-γ, TNF
Classic examples	Hay fever, food allergy, anaphylaxis, asthma	ABO transfusion reaction, Rh disease, AIHA, Goodpasture	Serum sickness, Arthus reaction, SLE, post-strep GN	Contact dermatitis, tuberculin test, chronic transplant rejection
Transferable by	Serum (IgE)	Serum (IgG/IgM)	Serum (immune complexes)	T cells only

Sensitization vs re-exposure in Type I allergy

Property	Sensitization (first exposure)	Re-exposure (effector phase)
Symptoms	None — clinically silent	Rapid: itch, hives, wheeze, shock
Dominant cells	Dendritic cell, Th2 CD4 T cell, B cell	Mast cell, basophil, later eosinophil
Key cytokines / signals	IL-4, IL-13 drive IgE class-switch	Allergen cross-links IgE on FcεRI
Molecular event	IgE synthesized and loaded onto FcεRI	Lyn/Syk → PLC-γ → Ca²⁺ → degranulation
Output	Armed, IgE-coated mast cells	Histamine, tryptase, leukotrienes, PGD2
Timescale	Days to weeks	Seconds to minutes (early); 4–8 h (late phase)

Famous experiments and history

• Richet and Portier discover anaphylaxis (1902). Aboard Prince Albert I of Monaco's yacht, Charles Richet and Paul Portier injected dogs with sea-anemone (Physalia, then Actinia) toxin. A dose harmless the first time killed the dog on a second, smaller injection weeks later. Richet coined anaphylaxis — "against protection" — for this paradoxical sensitization and won the 1913 Nobel Prize in Physiology or Medicine.
• Prausnitz and Küstner transfer allergy in serum (1921). Carl Prausnitz injected his own skin with serum from his fish-allergic colleague Heinz Küstner, then challenged the site with fish extract — and a wheal appeared. The "P-K reaction" proved a transferable serum factor ("reagin") carried immediate allergy, decades before that factor was identified as IgE. (The test was later abandoned for hepatitis risk.)
• Gell and Coombs classify hypersensitivity (1963). Philip Gell and Robin Coombs, in Clinical Aspects of Immunology, split hypersensitivity into four mechanistic types (I–IV) by effector — a framework still taught unchanged more than 60 years later.
• The Ishizakas identify IgE (1966–1967). Kimishige and Teruko Ishizaka in Denver purified the "reaginic" activity from allergic serum and showed it belonged to a new immunoglobulin class they named IgE; Gunnar Johansson and Hans Bennich in Uppsala independently characterized an IgE myeloma protein. IgE proved to be the rarest serum immunoglobulin (~0.05 µg/mL) yet the master switch of immediate allergy.
• Strachan proposes the hygiene hypothesis (1989). David Strachan noted in the BMJ that British children with more older siblings had less hay fever and eczema, suggesting early-life microbial exposure protects against allergy — a still-debated but influential explanation for why allergy rose with industrialization.
• The LEAP trial rewrites feeding advice (2015). The Learning Early About Peanut Allergy trial (Du Toit et al., NEJM) randomized high-risk infants to early peanut introduction or avoidance and found early introduction cut peanut allergy by roughly 80%, reversing decades of "avoid to prevent" guidance and reframing allergy as partly a failure of oral tolerance.