When does phagocytosis occur?

Phagocytosis is a biological process that occurs when certain types of cells engulf and consume foreign particles. In multicellular organisms, phagocytosis plays a crucial role in the immune system by engulfing and consuming microbial pathogens such as bacteria and viruses. It also plays a role in cellular homeostasis by engulfing and ingesting infected and apoptotic cells. 

The process of phagocytosis is induced when a cell, usually an immune system cell, is attempting to destroy and eliminate a superfluous substance such as a pathogenic microbe or an infected cell. During the process, specialized immune cells called phagocytes detect the presence of pathogens like bacteria or viruses. When these phagocytes recognize a threat, phagocytic receptors bind to ligands on the substance to be eliminated. Simultaneously, the cell membrane forms protrusions that surround and engulf the target particle, drawing it inwards.  

For phagocytosis to occur, an essential requirement is that the cell must be in physical contact with the particle it wants to engulf. In unicellular organisms, phagocytosis plays a nutritional role.