In cell biology, phagocytosis (from Ancient Greek φαγεῖν (phagein) , meaning "to devour", κύτος, (kytos) , meaning "cell", and -osis, meaning "process") is the process by which a cell—often a phagocyte or a protist—engulfs a solid particle to form an internal vesicle known as a phagosome. Phagocytosis was revealed by Canadian physician William Osler and later studed by Élie Metchnikoff.Phagocytosis is a specific form of endocytosis involving the vesicular internalization of solids such as bacteria, and is therefore distinct from other forms of endocytosis such as the vesicular internalization of various liquids (pinocytosis). Phagocytosis is involved in the acquisition of nutrients for some cells. The process is homologous to eating at the level of single-celled organisms; in multicellular animals, the process has been adapted to eliminate debris and pathogens, as opposed to taking in fuel for cellular processes, except in the case of the animal Trichoplax.In the immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris. For example, when a macrophage ingests a pathogenic microorganism, the pathogen becomes trapped in a phagosome which then fuses with a lysosome to form a phagolysosome. Within the phagolysosome, enzymes and toxic peroxides digest the pathogen. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytized.