CORPSE CLEARANCE IN APOPTOSIS
The apoptotic bodies are removed by phagocytosis. In C. elegans , neighboring cells take up the apoptotic bodies. In mammals, the situation is more complex. In a few areas of the body, neighboring cells engulf the apoptotic bodies. In most cases though, apoptotic bodies are engulfed by macrophages ( Fig. 20.17 ). Macrophages are cells of the immune system whose primary role is to digest anything foreign, such as invading bacteria. Normally macrophages recruit other immune cells to the site of infection. However, when dealing with apoptotic bodies, they do not recruit other immune cells. Thus macrophages can distinguish an apoptotic body from a foreign invader.
How does the macrophage know that apoptotic bodies do not require an immune response? Normally, when macrophage ingest bacteria, they secrete soluble proteins to recruit other mmune cells. Apparently apoptotic bodies have molecules on the surface that trigger the macrophage to digest them but without secreting the immune signaling factors. The numbers and mechanisms of “eat me” receptors are quite complex since different mammalian tissues may have different mechanisms to activate the macrophage. One such molecule appears to be phosphatidylserine, a phospholipid normally found only in the inner leaflet of the cell membrane ( Fig. 20.18 ). During apoptosis phosphatidylserine may be translocated to the outer leaflet. Macrophages have receptors for phosphatidylserine, allowing them to recognize the apoptotic body as “self.”
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