Type I Hypersensitivity: Allergic Reactions and IgE

Core Idea

Type I hypersensitivity (allergy) results from IgE-mediated mast cell and basophil activation. Th2-skewed responses to allergens (pollen, peanuts, dust mites) generate IgE antibodies that bind Fc receptors on mast cells. Cross-linking of IgE by allergens triggers rapid mast cell degranulation releasing histamine, tryptase, and lipid mediators causing vasodilatation, smooth muscle contraction, and increased vascular permeability within seconds. Repeated exposure can lead to anaphylaxis.

How It's Best Learned

Diagram Th2→IgE→mast cell sensitization. Explain mast cell degranulation signaling and the rapid kinetics. Compare localized allergies (seasonal rhinitis) with systemic anaphylaxis.

Common Misconceptions

• Allergies are rare (IgE-mediated allergies affect 20-30% of populations in developed countries). - Type I hypersensitivity always causes severe systemic reactions (localized reactions to harmless allergens are common).

Explainer

From your study of antibody isotypes, you know that IgE is the least abundant immunoglobulin in the blood but has the highest affinity for its Fc receptor. Type I hypersensitivity is what happens when the IgE system — originally evolved to combat parasitic worms — misfires against harmless environmental substances like pollen, pet dander, or peanut proteins. Understanding this pathway means following a two-phase process: sensitization first, then reaction on re-exposure.

During sensitization, a person inhales, ingests, or contacts an allergen for the first time. Antigen-presenting cells process the allergen and present peptide fragments to naive CD4+ T cells. In susceptible individuals, the immune response skews toward a Th2 profile, producing cytokines like IL-4 and IL-13 that drive B cells to undergo class switching to IgE. The resulting IgE antibodies circulate briefly, then bind tightly to FcεRI receptors on the surface of mast cells and basophils. At this point, these cells are "armed" — coated with allergen-specific IgE — but nothing happens yet. The person feels no symptoms during sensitization.

The reaction occurs on re-exposure. When the same allergen enters the body again, it binds to the IgE molecules already sitting on the mast cell surface. Because each allergen molecule has multiple epitopes, it can cross-link two or more adjacent IgE-FcεRI complexes, pulling them together on the membrane. This cross-linking triggers a rapid signaling cascade inside the mast cell, leading to degranulation — the explosive release of preformed granules containing histamine, tryptase, and heparin. The entire process from allergen contact to mediator release takes seconds to minutes, which is why allergic reactions are called immediate hypersensitivity.

The released mediators produce the familiar symptoms of allergy. Histamine causes vasodilation (redness), increased vascular permeability (swelling), and smooth muscle contraction (bronchoconstriction in asthma, cramping in food allergies). Mast cells also synthesize new lipid mediators — prostaglandins and leukotrienes — that sustain and amplify the inflammatory response over hours. When the reaction stays localized, you get hay fever, hives, or mild GI distress. When allergen enters the bloodstream and triggers widespread mast cell degranulation simultaneously, the result is anaphylaxis: a life-threatening drop in blood pressure, airway constriction, and potential cardiovascular collapse that requires immediate epinephrine treatment. The difference between a runny nose and anaphylaxis is not a different mechanism — it is the same IgE-mast cell pathway operating at different scales.