Clonal mast-cell disease
This is a mast-cell system failure. The cells are genetically abnormal, over-numerous, and they release the wrong mediators in the wrong proportions.
This is typically due to an activating mutation in KIT (D816V) or an increased copy number of TPSAB1.
| Normal mast cell | Clonal mast cell | |
|---|---|---|
| Activation threshold | High | Abnormally low |
| Mast-cell burden | Normal | Massively increased |
| Degranulation pattern | Histamine-dominant | Tryptase / PAF / PGD₂ dominant |
| Localization | Skin-heavy | Bone marrow & vasculature-heavy |
| Control signals | Intact | Autonomous activation |
Because clonal mast cells are:
- Hyper-reactive
- Autonomous
- Numerous
Stimuli such as:
- Temperature change
- Exercise
- Insect sting
- Alcohol
- Contrast dye
That are trivial in normal people can activate millions of mast cells at once. The reaction magnitude reflects the large number of mast-cells, not allergen dose.
Normal Anaphylaxis Reaction
Histamine predominates the reaction:
| Feature | Why it happens |
|---|---|
| Pruritus, hives, flushing, angioedema | Histamine release into the skin |
| Wheeze | Bronchial histamine + leukotrienes |
| Hypotension | Late / secondary effect |
Clonal mast-cell anaphylaxis (mastocytosis, clonal MCAS)
These patients have:
- Massive mast-cell burden
- Abnormal mediator release profile
- Preferential release of tryptase, prostaglandins, and PAF
- Less histamine into the skin
| Feature | Mechanism |
|---|---|
| Severe hypotension / syncope | Platelet-activating factor (PAF) + prostaglandin (D₂) |
| Tryptase | Endothelial barrier disruption |
| Minimal or absent hives | Little cutaneous histamine |
| Often no itching | Due to relatively little histamine into skin |
| Sudden collapse | Vascular catastrophe |
Why epinephrine often “doesn’t work well”
This physiology is less responsive to epinephrine than histamine-mediated allergy, which is why these patients have refractory hypotension and higher fatality rates.
PAF-mediated shock causes:
- Severe vascular leak
- Myocardial depression
- Distributive collapse