**3.2 Activation of natural immunity**

Macrophages participate in natural immunity by engulfing and digesting pathogens. They protect hosts from the infections and damages through phagocytosis [8]. Macrophages are the first defence against pathogens working with neutrophils and are specific phagocytes with long life. After the invasion of pathogens, neutrophils are firstly recruited to the site of infection, die after phagocytosis of pathogens, and generate neutrophil traps (NETs). Then, macrophages are recruited and digest NETs after approximately 48 hours later. Recruited macrophages digest pathogens and dead cells through phagocytosis. Finally, they initiate immune responses through releasing factors like TNF-α to recruit other immune cells such as lymphocytes.
