degranulate

Degranulation is the process by which secretory granules stored inside certain cells fuse with the plasma membrane and release their contents into the surrounding space or into a phagosome. It is a key mechanism in immune defense and in inflammatory responses. The term covers both full degranulation, where most or all granule contents are released, and piecemeal degranulation, a more gradual, selective release observed in some cells.

In humans, degranulation is most often associated with mast cells and basophils, which release histamine, proteases such as tryptase and chymase, and proteoglycans that contribute to vascular permeability and smooth muscle contraction. Eosinophils and neutrophils also undergo degranulation, releasing cytotoxic proteins, enzymes, and antimicrobial peptides stored in their granules; the specific contents reflect the cell type and the context of activation.

Mechanistically, degranulation involves calcium signaling and membrane fusion mediated by SNARE proteins and other regulatory factors. Mast cells and basophils can undergo either full exocytosis or piecemeal degranulation, in which granule contents are delivered to the cell surface via vesicles without complete granule rupture. Activation can occur through receptors such as Fc epsilon RI during allergic responses, as well as through complement receptors, cytokine receptors, and pattern-recognition receptors (TLRs) during infection.

Physiological roles include rapid defense against pathogens and modulation of vascular tone and tissue repair. Pathological degranulation contributes to allergic diseases (asthma, anaphylaxis) and chronic inflammatory conditions, where excessive mediator release causes tissue damage. Therapies targeting degranulation—such as mast cell stabilizers, antihistamines, and anti-IgE treatment—aim to reduce mediator release and its clinical consequences.