Fig. 2. Interaction between macrophage and apoptotic cell mediates phagocytosis and efferocytosis of macrophages.

As PtdSer is flipped onto the surface of dying cells, TIM4 on macrophages can identify and bind to PtdSer tightly. However, the binding cannot initiate engulfment. To engulf dying cells, TAM family, mainly MerTK and AXL, is required to bind with GAS6 or PROS1 in order to initiate rapid engulfment activity.