Medicine & USMLE

Type 1 Hypersensitivity Reactions

Hypersensitivity Reactions
  1. Type 1 Hypersensitivity Reactions
  2. Type 2 Hypersensitivity Reactions
  3. Type 3 Hypersensitivity Reactions
  4. Type 4 Hypersensitivity Reactions


Type I Hypersensitivity Reactions (T1HSR) are a type of unwanted immune response, caused by pre-formed IgE antibodies against harmless allergens. T1HSR is more commonly known as allergy. Because it is caused by pre-formed antibodies, type 1 hypersensitivity reactions happen quickly, almost instantaneously after exposure. The resulting immune response is divided into two phases, the early phase, and the (less important) late phase.

The early phase reaction happens immediately after exposure and is caused by IgE binding to receptors on mast cells and basophils. The activation of mast cells and basophils releases histamine and other inflammatory mediators, which then act to cause the symptoms of allergy and anaphylaxis. The late phase reaction happens hours after exposure, and is mediated mostly by eosinophils. In the late te phase reaction is responsible for further inflammation and tissue damage. 

Key Points

  • Type I Hypersensitivity Reactions (T1HSR)
    • Undesired immune response to harmless antigens
    • Include allergy, anaphylaxis, and atopy
    • Mediated by preformed Immunoglobulin E (IgE) against an allergen
    • Two phases
      • Early phase
        • begins immediately after exposure
        • Mast Cells and Basophils bind IgE and degranulate
          • Releases histamine, heparin, and tryptase
        • Histamine causes urticaria (hives), anaphylaxis, and other symptoms of allergy
      • Late phase
        • happens hours after exposure
        • Chemokines attract Eosinophils
        • Inflammation and tissue damage