Granulated connective tissue cells releasing histamine on IgE cross-linking
Expansion
Appearance: large oval cells with abundant cytoplasmic granules, found in connective tissue especially around blood vessels and beneath epithelial surfaces exposed to the environment, namely skin, airway and gut.
Staining: granules show metachromasia, appearing purple with toluidine blue because the densely charged heparin alters the dye’s absorption. They also stain with Giemsa and are identified by CD117 (c-KIT) and tryptase immunohistochemistry.
Granule contents
- Histamine: vasodilatation, increased permeability, bronchoconstriction, itch
- Heparin
- Tryptase and chymase: a raised serum tryptase confirms mast cell degranulation and is used to confirm anaphylaxis retrospectively
- Newly synthesised prostaglandins and leukotrienes
Activation: cross-linking of surface IgE by antigen is the classic route, underlying type I hypersensitivity. Non-IgE routes include complement C3a and C5a, opiates, vancomycin, contrast media, physical stimuli and cold.
Mastocytosis is clonal mast cell proliferation, giving urticaria pigmentosa with a positive Darier sign (wealing on rubbing a lesion) and, in systemic disease, flushing, anaphylaxis and organ infiltration with a persistently raised tryptase.