The Science Behind Allergies: How Immune Hypersensitivity Works

Why Allergies Happen: Sensitization and the IgE Response

An allergic reaction requires two separate exposures to an allergen. During the first exposure, called sensitization, the allergen enters the body through the skin, respiratory tract, or gastrointestinal tract and is captured by dendritic cells, which process it and present peptide fragments to naive CD4+ T cells in the draining lymph nodes. In individuals genetically predisposed to allergy, these T cells differentiate into Th2 helper cells rather than the Th1 cells that would drive a normal pathogen-clearing response. Th2 cells produce the cytokines IL-4, IL-5, and IL-13, which collectively instruct B cells to undergo class switching to IgE, the antibody class specialized for allergic responses.

The IgE antibodies produced during sensitization circulate briefly in the blood before binding with extremely high affinity (Ka approximately 10^10 M^-1) to the FcepsilonRI receptor on the surface of mast cells in tissues and basophils in the blood. A single mast cell can carry 100,000 to 500,000 IgE molecules on its surface, each one specific for a different allergen epitope. At this stage, the person is sensitized but has experienced no symptoms. The loaded mast cells sit in tissues for weeks to months, waiting for a second encounter with the allergen.

Upon re-exposure, the allergen cross-links two or more IgE molecules on the mast cell surface by binding to their antigen-binding sites simultaneously. This cross-linking triggers a rapid signaling cascade inside the mast cell that results in degranulation, the explosive release of preformed granules containing histamine, heparin, tryptase, and other mediators, within seconds. The mast cell also begins synthesizing and releasing lipid mediators (prostaglandins and leukotrienes) and cytokines (TNF-alpha, IL-4, IL-13) over the following minutes to hours, amplifying and sustaining the inflammatory response.

What Histamine and Other Mediators Do

Histamine is the most well-known allergic mediator and is responsible for many of the immediate symptoms of allergic reactions. It acts on H1 receptors on blood vessel endothelial cells to cause vasodilation (redness) and increased vascular permeability (swelling and edema). In the airways, histamine contracts bronchial smooth muscle, causing the airway narrowing characteristic of asthma attacks. In the nasal mucosa, histamine stimulates mucus production and nerve endings, causing congestion, runny nose, and sneezing. In the skin, histamine causes the characteristic wheal-and-flare reaction of hives (urticaria), with a central pale swelling surrounded by a red halo.

Leukotrienes, particularly leukotriene C4, D4, and E4 (collectively called the slow-reacting substance of anaphylaxis), are 100 to 1,000 times more potent than histamine at contracting airway smooth muscle. They are the primary drivers of sustained bronchoconstriction in asthma and are the target of leukotriene receptor antagonists like montelukast (Singulair). Prostaglandin D2, the major prostanoid produced by mast cells, causes vasodilation, bronchoconstriction, and neutrophil chemotaxis.

The late-phase allergic response occurs 4 to 8 hours after the initial reaction and is driven by eosinophils, basophils, and Th2 cells recruited to the site by chemokines released during the immediate reaction. Eosinophils release major basic protein, eosinophil peroxidase, and eosinophil cationic protein, all of which are toxic to airway epithelium and contribute to the chronic tissue damage seen in persistent asthma and allergic rhinitis. This late-phase response explains why allergy symptoms often recur hours after the initial exposure, even without additional allergen contact.

The Four Types of Hypersensitivity

Immunologists classify harmful immune responses into four types of hypersensitivity, first described by Philip Gell and Robin Coombs in 1963. Only Type I involves IgE and is what most people mean by "allergy," but all four types represent immune responses that cause tissue damage rather than protection.

Type I (immediate) hypersensitivity is the classic allergic reaction described above: IgE-mediated mast cell degranulation in response to environmental allergens. It occurs within minutes of allergen exposure and includes hay fever, food allergies, asthma, and anaphylaxis. Atopy, the genetic tendency to produce IgE against common environmental allergens, affects 30 to 40 percent of the population in Western countries.

Type II (cytotoxic) hypersensitivity occurs when IgG or IgM antibodies bind to antigens on the surface of host cells, marking them for destruction by complement, phagocytes, or NK cells. Examples include hemolytic disease of the newborn (anti-Rh antibodies attacking fetal red blood cells), autoimmune hemolytic anemia, and transfusion reactions from ABO blood group incompatibility. The damage is directed at specific cell types that carry the target antigen.

Type III (immune complex) hypersensitivity results from the deposition of antigen-antibody complexes in tissues, where they activate complement and recruit neutrophils that release tissue-damaging enzymes. Serum sickness, caused by the injection of foreign proteins like anti-venom, is the classic example. Immune complex deposition in the kidneys, joints, and blood vessels also drives much of the pathology in systemic lupus erythematosus. The Arthus reaction, a localized form of Type III hypersensitivity, can occur at injection sites when pre-existing antibodies react with an injected antigen.

Type IV (delayed-type) hypersensitivity is mediated by T cells rather than antibodies and takes 24 to 72 hours to develop. Contact dermatitis from poison ivy, nickel, or latex is a Type IV reaction, as is the tuberculin skin test (Mantoux test), in which a localized induration develops 48 to 72 hours after injection of tuberculin antigens into the skin of someone previously exposed to Mycobacterium tuberculosis. Organ transplant rejection is also primarily a Type IV response. The delayed timeline reflects the time required for T cells to migrate to the site, recognize antigen, and recruit inflammatory cells.

Common Allergens and Why They Trigger Reactions

Not all proteins are equally allergenic, and the molecular properties that make certain substances effective allergens are partially understood. Most allergens are proteins or glycoproteins with molecular weights between 10 and 70 kilodaltons, small enough to diffuse through mucosal barriers but large enough to cross-link IgE on mast cells. Many allergens have enzymatic activity: the major house dust mite allergen Der p 1 is a cysteine protease that cleaves tight junction proteins in airway epithelium, facilitating allergen penetration and activating innate immune cells through protease-activated receptors.

Pollen allergens account for the largest share of respiratory allergies globally. The major allergens of birch pollen (Bet v 1), grass pollen (Phl p 1 and Phl p 5), and ragweed pollen (Amb a 1) are among the most thoroughly characterized allergenic proteins. Pollen seasons are extending due to climate change: ragweed pollen season in North America has lengthened by 13 to 27 days compared to 1995 levels, and pollen concentrations have increased by 21 percent in the same period.

Food allergies affect approximately 8 percent of children and 10 percent of adults in the United States, and their prevalence has increased dramatically over the past three decades. The eight most common food allergens (peanuts, tree nuts, milk, eggs, wheat, soy, fish, and shellfish) account for 90 percent of allergic reactions. Peanut allergy is among the most dangerous because it is more likely than other food allergies to cause anaphylaxis and less likely to be outgrown. The LEAP study (Learning Early About Peanut Allergy) demonstrated that early introduction of peanut-containing foods to high-risk infants between 4 and 11 months of age reduced peanut allergy prevalence by 81 percent compared to avoidance, fundamentally changing clinical recommendations for allergy prevention.

Anaphylaxis: The Most Severe Allergic Reaction

Anaphylaxis is a severe, whole-body allergic reaction that can be fatal within minutes if untreated. It occurs when massive, systemic mast cell degranulation releases histamine and other mediators throughout the body simultaneously, causing widespread vasodilation, a catastrophic drop in blood pressure, airway constriction, and cardiovascular collapse. The incidence of anaphylaxis in the United States is estimated at 30 to 60 cases per 100,000 person-years, with food, insect venom, and medications being the most common triggers.

Epinephrine (adrenaline) is the first-line treatment for anaphylaxis and should be administered immediately via intramuscular injection into the outer thigh. Epinephrine acts on alpha-1 adrenergic receptors to constrict blood vessels and raise blood pressure, on beta-1 receptors to increase heart rate and cardiac output, and on beta-2 receptors to relax bronchial smooth muscle and open the airways. It also stabilizes mast cells and prevents further degranulation. The auto-injector devices (EpiPen, Auvi-Q) deliver a pre-measured dose and are designed for self-administration by patients or bystanders. Delayed epinephrine administration is the strongest predictor of fatal anaphylaxis, making immediate access to auto-injectors essential for anyone with a history of severe allergic reactions.

Allergy Treatments and Immunotherapy

Conventional allergy treatment focuses on allergen avoidance, symptom control with antihistamines and corticosteroids, and mast cell stabilizers. Second-generation antihistamines (cetirizine, loratadine, fexofenadine) block H1 receptors without crossing the blood-brain barrier, providing symptom relief without the sedation caused by first-generation antihistamines like diphenhydramine. Intranasal corticosteroids (fluticasone, mometasone) reduce nasal inflammation, congestion, and mucus production and are the most effective single treatment for allergic rhinitis.

Allergen immunotherapy (AIT), commonly called allergy shots, is the only treatment that modifies the underlying immune response rather than just managing symptoms. Subcutaneous immunotherapy (SCIT) involves injecting gradually increasing doses of allergen extract over a build-up period of 3 to 6 months, followed by monthly maintenance injections for 3 to 5 years. The treatment works by shifting the immune response away from IgE-driven Th2 pathways toward IgG4 production and regulatory T cell expansion. IgG4 antibodies compete with IgE for allergen binding but do not trigger mast cell degranulation, effectively blocking the allergic cascade. AIT is effective for pollen, dust mite, mold, and insect venom allergies, with benefits persisting for years after treatment discontinuation.

Sublingual immunotherapy (SLIT), in which allergen tablets or drops are placed under the tongue daily, offers a needle-free alternative that can be taken at home after the first dose. FDA-approved SLIT tablets are available for grass pollen (Grastek), ragweed pollen (Ragwitek), and house dust mite (Odactra). SLIT has a lower risk of systemic reactions than SCIT but may be somewhat less effective for some allergens.

Biologic therapies represent the newest frontier. Omalizumab (Xolair), an anti-IgE monoclonal antibody, binds free IgE in the blood and prevents it from attaching to mast cells, effectively disarming the allergic response at its source. It is approved for moderate-to-severe allergic asthma, chronic idiopathic urticaria, and, as of 2024, food allergy reduction. Dupilumab (Dupixent), which blocks IL-4 and IL-13 signaling through the shared IL-4 receptor alpha subunit, is approved for atopic dermatitis, asthma, chronic rhinosinusitis with nasal polyps, and eosinophilic esophagitis. These biologics target the Th2 inflammatory axis with precision and have produced dramatic improvements in quality of life for patients with severe allergic disease.