Hypersensitivity, Immediate

**Definition:** Hypersensitivity reactions which occur within minutes of exposure to challenging antigen due to the release of histamine which follows the antigen-antibody reaction and causes smooth muscle contraction and increased vascular permeability.

**Tree numbers:** - C20.543.480

**Synonyms:** - Atopic Hypersensitivity - Hypersensitivity, Atopic - Hypersensitivity, Type I - Type I Hypersensitivity - IgE-Mediated Hypersensitivity