Phagocytosis by zippers and triggers.

Two mechanisms have been considered for particle phagocytosis. According to the zipper mechanism, ingestion occurs by sequential engagement of a phagocyte's membrane against the particle surface, and pseudopod advance proceeds no further than receptor-ligand interactions permit. In contrast, in the trigger mechanism particle binding initiates an all-or-none phagocytic response. Although the weight of experimental evidence has favoured the zipper mechanism, recent observations of bacterial entry into epithelial cells and macrophages indicate an indiscriminate, triggered response. This prompts a reconsideration of the underlying mechanisms.