Mast Cell Mediators

When a mast cell undergoes Degranulation, it doesn’t just release Histamine. It releases a cocktail of chemically distinct molecules, each with different targets and timelines. This is why antihistamines alone often provide incomplete relief in MCAS — they address one mediator while others continue causing symptoms.

Pre-Formed Mediators (Released in Seconds)

These are stored ready-to-go in granules.

Histamine — Vasodilation, vascular permeability, bronchoconstriction, gastric acid secretion, nerve stimulation, wakefulness. The most abundant mediator by volume. See the full Histamine note for receptor-specific effects.

Tryptase — A protease (protein-cutting enzyme) that degrades extracellular matrix, activates other immune pathways, and contributes to tissue remodeling. Clinically important because it’s the primary lab marker for mast cell activation (see Serum Tryptase). However, it is not always elevated in MCAS, particularly during piecemeal degranulation.

Heparin — An anticoagulant (prevents blood clotting). Stored in granules as a scaffold for other mediators. Once released, it can cause easy bruising, heavy menstrual bleeding, and interfere with wound healing. Heparin release also contributes to the bone loss sometimes seen in severe MCAS.

Proteases (chymase, carboxypeptidase A3) — Break down proteins in the local tissue environment. Can damage the gut lining and contribute to Intestinal Permeability. Chymase also converts angiotensin I to angiotensin II, which can affect blood pressure — relevant to POTS.

TNF-α (pre-stored fraction) — A pro-inflammatory cytokine. Some is stored in granules for immediate release; more is synthesized later.

Newly Synthesized Mediators (Minutes to Hours)

After the initial granule dump, the cell starts manufacturing additional mediators.

Lipid Mediators (from Arachidonic Acid)

When a mast cell is activated, phospholipase A2 cleaves arachidonic acid from the cell membrane. This fatty acid is then processed by two competing enzyme systems:

COX Enzymes pathway → Prostaglandins

• PGD2 (Prostaglandin D2) — The dominant prostaglandin from mast cells. Causes vasodilation, bronchoconstriction, and is a potent driver of flushing. Also involved in sleep regulation. PGD2 is measured in 24-Hour Urine Testing as its metabolite 11β-PGF2α.
• PGE2 — Pro-inflammatory, sensitizes pain receptors, causes fever.

5-LOX pathway → Leukotrienes

• LTC4, LTD4, LTE4 (cysteinyl leukotrienes) — Potent bronchoconstrictors (100-1000x more potent than histamine at constricting airways). Also increase vascular permeability and mucus production. LTE4 is measured in urine. This is the pathway that Montelukast targets.

Why this matters

Salicylates and NSAIDs interact with these pathways by inhibiting COX enzymes, which can shunt more Arachidonic Acid through the 5-LOX pathway, increasing leukotriene production. This is why aspirin and NSAIDs can worsen symptoms in some mast cell patients.

Cytokines

Cytokines are signaling proteins that coordinate the broader immune response. Mast cells produce a wide array:

• IL-1β, IL-6 — Pro-inflammatory, contribute to systemic symptoms like fatigue, malaise, fever-like feelings
• IL-13 — Promotes mucus production, involved in airway inflammation
• TNF-α — Amplifies inflammation, recruits other immune cells
• IL-4, IL-5 — Drive Th2 immune responses, promote IgE production (which creates more mast cell triggers — a positive feedback loop)
• IL-33 — An “alarmin” that directly activates mast cells, contributing to the self-perpetuating cycle

Other

• Chemokines — Recruit neutrophils, eosinophils, and other immune cells to the area
• Vascular endothelial growth factor (VEGF) — Promotes new blood vessel formation, relevant to chronic inflammation
• Nerve growth factor (NGF) — Promotes nerve sprouting, may contribute to pain sensitization and the development of Small Fiber Neuropathy

The Combination Effect

No single mediator explains the full clinical picture of MCAS. The symptom experience is produced by the combination:

• Histamine + Prostaglandins → flushing is more severe than either alone
• Histamine + Leukotrienes → bronchoconstriction compounds
• Cytokines + Histamine → brain fog and fatigue layer onto acute symptoms
• Heparin + local tissue damage → bruising and bleeding issues

This is why treatment often requires a multi-targeted approach rather than just H1 Antihistamines. See Medications Overview.

Downstream: What You Feel

See Symptom Mapping for a detailed breakdown of which mediators produce which symptoms and why.