Type 1 Hypersensitivity and Allergies

Mar 15, 2025

Hypersensitivity and Allergic Reactions

Overview of Hypersensitivity

  • Hypersensitivity occurs when the immune system reacts in a way that damages the body.
  • There are four types of hypersensitivities.

Type I Hypersensitivity

  • Type I Hypersensitivity is mediated by Immunoglobulin E (IgE) antibodies.
  • Also known as IgE-mediated hypersensitivities or immediate hypersensitivities because reactions happen in minutes.
  • Most allergic reactions are IgE-mediated.

Allergy

  • Derived from Greek: "Allos" (other) and "ergon" (reactivity).
  • Reactions to external molecules (allergens) to which most people don’t react.
  • Common allergens: foods, animal dander, bee stings, mold, drugs, pollen, latex, lotions, and soaps.

Mechanism of Allergic Reaction

  1. Sensitization Phase

    • First exposure to an allergen leads to sensitization.
    • Genetic predisposition plays a role—T-helper cells are hypersensitive to certain antigens.
    • Antigen-presenting cells (e.g., dendritic cells, macrophages) present antigens to T-helper cells.
    • Naive T-helper cells convert to TH2 cells in presence of interleukins (IL-4, IL-5, IL-10).
    • TH2 cells stimulate B cells to switch from IgM to IgE antibodies specific to allergen.
    • IgE antibodies bind to Fc epsilon receptors on mast cells.

  2. Subsequent Exposure

    • Upon second exposure, mast cells bind to the antigen leading to degranulation.
    • Release of pro-inflammatory molecules, including histamine.
    • Effects include bronchial smooth muscle contraction, blood vessel dilation, increased permeability (edema, urticaria).

Early and Late Phase Reactions

  • Early Phase (minutes after exposure): Release of histamine and other mediators.
  • Late Phase (8-12 hours after exposure): Involvement of immune cells (TH2, eosinophils, basophils) due to cytokines and leukotrienes.

Symptoms of Allergic Reactions

  • Mild: Hives, eczema, allergic rhinitis, asthma.
  • Severe: Anaphylactic shock due to increased vascular permeability and airway constriction.

Treatment

  • Antihistamines to block histamine effects.
  • Corticosteroids to reduce inflammation.
  • Epinephrine (EpiPen) for severe reactions:
    • Constricts blood vessels.
    • Prevents anaphylactic shock.

Medical Attention

  • Severe reactions require immediate medical attention as symptoms can improve temporarily and then worsen.

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