When a macrophage engulfs a bacterium, it triggers a stress pathway in the endoplasmic reticulum (1). This in turn stimulates mitochondria to produce reactive oxygen species (2), which are packaged into vesicles and shuttled to the phagosome (3). There, the damaging molecules are thought to aid in killing the pathogen. Finally, the bacterial remains are degraded once the phagosome fuses with a lysosome (4).
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