Anaphylactic and atopic – two phases:- Immediate (minutes): antigen crosslinks preformed IgE on presensitized mast cells → immediate degranulation → release of histamine (a vasoactive amine) and tryptase (a marker of mast cell activation).- Late (hours): chemokines (attract inflammatory cells, eg, eosinophils) and cytokines (eg, leukotriens) from mast cells → inflammation and tissue damage.

Test: skin test or blood test (ELISA) for allergen-specific IgE.

Example:- Anaphylaxis (eg, food, drug, or bee sting allergies)