Type I following the Gell and Coombs Classification . Allergic reactions are classified depending on the type of tissue damage that develops. A Type I reaction occurs when an antigen entering the body encounters mast cells or basophils . These types of immune defense cells have antibodies attached to their surfaces and the antigen binds to this. The result is the release of histamine , causing blood vessel dilation and a narrowing of the airways .

Examples include :

  Allergic Rhinitis
  Intrinsic Asthma
  Anaphylactic Shock