Mast Cell Degranulation

Mast cells are long-lived granulocytes resident in skin, gut and airway mucosa, packed with granules. Degranulation is the fusion of those granules with the plasma membrane, emptying histamine, tryptase, chymase and heparin into tissue within seconds; a slower wave of newly made prostaglandin D2 and leukotrienes follows over minutes. The classical trigger is antigen cross-linking IgE on the high-affinity receptor, which requires prior sensitisation. A second route runs through MRGPRX2, which responds to basic, cationic molecules with no antibody involvement at all.

That second route is what makes the term relevant to peptides. MRGPRX2 is activated by a diverse set of cationic drugs, among them vancomycin, several opioids and neuromuscular blockers, and by cationic amphipathic peptides generally. Clinically the reaction looks like allergy: flushing, wheal and flare, hypotension. Serum tryptase, peaking roughly one to two hours after a systemic event, is the laboratory footprint of degranulation whichever receptor started it.

The distinction changes what a reaction implies. An IgE-mediated event predicts escalating severity on re-exposure and is testable by specific IgE or skin testing. An MRGPRX2-mediated event is concentration and rate dependent, can occur on first exposure, and implies no sensitisation. Calling every immediate reaction an allergy over-restricts future use and hides a dose-rate problem.

The error found in practice is inferring that a first-exposure reaction rules out an immune mechanism, or that any wheal at an injection site proves allergy. Cationic peptide preparations can provoke mast cell activation as a direct pharmacological property, and a persistently raised baseline tryptase points to a clonal mast cell disorder rather than to whatever was injected.