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[″ + ″ + osis, condition]
A three-stage process by which phagocytes (neutrophils, monocytes, and macrophages) engulf and destroy microorganisms, other foreign antigens, and cell debris. Generally, these substances must be covered with opsonins, such as antibodies or complement, to initiate binding with cell receptors on the phagocytes, the first stage in phagocytosis. In the second stage, the particle is engulfed and enclosed in a vacuole (phagosome). During the third stage, the phagosome merges with lysosomes whose enzymes destroy the engulfed particle.
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PHAGOCYTOSIS 1) Phagocyte ingests & destroys bacteria; 2) phagocyte encounters bacteria & sends out membrane projections (pseudopodia); 3) pseudopodia envelop organism, forming sac (phagosome); 4) phagosome travels to inner cell & fuses with lysosome; 5) lysosome's digestive enzymes destroy bacteria; 6) waste products released. ; SEE: defensin; SEE: lysozyme; SEE: macrophage; SEE: neutrophil; SEE: oxygen radical
Most bacteria are killed during phagocytosis by oxygen radicals, which are formed during the respiratory burst when phagosomes and lysosomes merge. When oxygen radical production is excessive, tissue damage occurs. Lysozymes, defensins, and bacteriocidal permeability-increasing (BPI) protein also destroy bacteria and other organisms; their actions do not depend on the generation of oxygen radicals.

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