Mast Cell Identification in Tissue
Tryptase immunohistochemistry is the most reliable and sensitive method to identify mast cells in tissue sections, as it detects both normal and neoplastic mast cells with superior sensitivity compared to traditional metachromatic stains. 1
Primary Identification Method
Use tryptase immunohistochemistry as your first-line staining method for mast cell detection. 1, 2
- Tryptase is the most sensitive marker because it allows detection of small and/or immature mast cell infiltrates that may be missed by other methods 1
- This marker is expressed on all mast cells regardless of maturity or activation state 1, 2
- Tryptase staining is far superior to hematoxylin and eosin (H&E) alone, which is unreliable due to variable mast cell morphology and often fails to visualize mast cell granules 3
- Traditional metachromatic stains (toluidine blue) have been largely supplanted by immunohistochemistry due to inferior sensitivity and specificity 2, 4
Complementary Markers for Comprehensive Assessment
Combine tryptase with CD117 (c-kit) and CD25 immunohistochemistry on bone marrow core biopsies to optimize quantification of mast cell burden. 1
- CD117 (c-kit) is co-expressed with tryptase on all normal mast cells and provides confirmatory identification 1, 5
- CD25 is critical for distinguishing neoplastic from normal mast cells, as aberrant CD25 expression (with or without CD2) is a minor diagnostic criterion for systemic mastocytosis 1
- CD25 is more sensitive than CD2 for detecting neoplastic mast cells, since CD2 is only expressed in 50-60% of indolent systemic mastocytosis cases and is absent in advanced disease 1
Critical Limitation to Understand
Neither tryptase nor CD117 can distinguish between normal/reactive and neoplastic mast cells—you must add CD25 and/or CD2 to make this distinction. 1
- Tryptase and CD117 are constitutively expressed on all mast cells regardless of clonality 1
- Only aberrant expression of CD25 (and to a lesser extent CD2) identifies the neoplastic phenotype 1, 6
- CD30 may be helpful when CD25 is negative, particularly in well-differentiated systemic mastocytosis, but is considered optional 1
Practical Staining Protocol for Bone Marrow
On bone marrow core biopsies, perform this immunohistochemistry panel: tryptase, CD117, and CD25 as standard; add CD30 if CD25 is negative. 1
- This combination allows both identification and quantification of mast cell burden 1
- CD34 staining should also be obtained in suspected systemic mastocytosis with associated hematologic neoplasm to quantify myeloblast proportion 1
- Reticulin and collagen staining are necessary to assess bone marrow fibrosis grade (MF-0 to MF-3), which is common in advanced systemic mastocytosis 1
Alternative Methods (Lower Priority)
Cytochemical staining for aminocaproate esterase (chloroacetyl esterase) is the most specific enzyme marker for mast cells on cytologic specimens but is less practical than immunohistochemistry for tissue sections. 4, 5
- This enzyme histochemical method can be useful for bone marrow aspirate smears 4, 5
- It is more cumbersome than immunohistochemistry and has been largely replaced in routine practice 4
Common Pitfalls to Avoid
- Do not rely on H&E staining alone—mast cells often appear as nondescript cells with amphophilic cytoplasm and may be completely overlooked without specific staining 3
- Do not assume tryptase-positive cells are neoplastic—you must demonstrate aberrant CD25 expression to confirm systemic mastocytosis 1
- Do not use CD2 as your sole aberrant marker—it has lower sensitivity than CD25 and may be absent in advanced disease 1
- Avoid formalin-fixed tissue that has been rapidly decalcified—this yields unsatisfactory results for molecular testing, though EDTA decalcification is acceptable 1
Flow Cytometry as Complementary Tool
Flow cytometry with CD117, CD25, and CD2 serves as a complementary tool for mast cell characterization but requires specialized rare-event analysis techniques. 1