Mediate allergic reaction in local tissues. Involved in type I hypersensitivity reactions.

- Contain basophilic granules and originate from the same precursor as basophils but are not the same cell type.

- Can bind the Fc portion of IgE to membrane.

- Activated by tissue trauma, C3a and C5a, surface IgE crosslinking by antigen (IgE receptor aggregation) → degranulation → release of histamine, heparin, tryptase, and eosinophil chemotactic factors.

- Cromolyn sodium prevents mast cell degranulation (used for asthma prophylaxis).