Degranulation
Degranulation is the process by which a mast cell releases the chemical mediators stored inside it. It’s the central event in mast cell biology — everything upstream is about triggering it, everything downstream is about what the released chemicals do.
The Mechanics
Mast cells store pre-formed mediators in membrane-bound granules — small vesicles packed with chemicals ready for immediate deployment. When the cell receives an activation signal, these granules migrate to the cell surface, fuse with the outer membrane, and dump their contents into the surrounding tissue.
This is fast. The pre-formed mediators (especially Histamine and Tryptase) are released within seconds. There’s no synthesis delay — the weapons are pre-loaded.
Two Phases of Release
Immediate release (seconds to minutes): Pre-formed mediators stored in granules:
- Histamine
- Tryptase and other proteases
- Heparin
- TNF-α (some is pre-stored)
- Serotonin (in smaller amounts than rodent mast cells, but present)
Delayed synthesis (minutes to hours): After degranulation, the cell also begins manufacturing new mediators from scratch:
- Prostaglandins (especially PGD2) — synthesized from Arachidonic Acid via the COX Enzymes pathway
- Leukotrienes (especially LTC4) — synthesized from Arachidonic Acid via the 5-LOX pathway
- Cytokines — IL-1, IL-6, TNF-α, IL-13, and many others
- Chemokines — signals that recruit other immune cells
This two-phase release explains why mast cell reactions can have an immediate component and then a delayed wave hours later. A biphasic reaction isn’t two separate events — it’s one degranulation event with two temporal phases of mediator release.
Types of Degranulation
Not every activation event is an all-or-nothing explosion.
Full degranulation: Massive, rapid release of all granule contents. This is what happens in anaphylaxis. The cell essentially empties itself.
Piecemeal degranulation: Selective, partial release of specific mediators without complete granule fusion. The cell releases some Histamine but not all its Tryptase, for example. This is likely the dominant mode in MCAS — chronic, low-grade, selective release rather than dramatic anaphylactic events.
Differential release without degranulation: Some mediators (particularly Cytokines and Leukotrienes) can be released through secretory pathways that don’t involve granule fusion at all. The cell can produce inflammatory signals without technically degranulating.
Why this matters clinically
Piecemeal degranulation explains why someone with MCAS can have significant symptoms without elevated Serum Tryptase. Tryptase is the canonical marker of degranulation, but if the cell is selectively leaking Histamine and Prostaglandins without full granule dump, tryptase may remain normal. This is a major reason MCAS is underdiagnosed.
What Triggers It
- IgE-Mediated Activation — the classical allergy pathway
- Non-IgE Activation Pathways — a long and growing list of alternative triggers
- The HPA Axis and Mast Cells — stress hormones as direct triggers
- Estrogen and Mast Cells — hormonal priming of the degranulation threshold
What It Releases
See Mast Cell Mediators for a complete map of released chemicals and their downstream effects.