Phagocytosis

Phagocytosis (from Ancient Greek φαγεῖν (phagein) 'to eat' and κύτος (kytos) 'cell') is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is called a phagocyte.

In a multicellular organism's immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris. The ingested material is then digested in the phagosome. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytized. Some protozoa use phagocytosis as means to obtain nutrients. The two main cells that do this are the Macrophages and the Neutrophils of the immune system.

Where phagocytosis is used as a means of feeding and provides the organism part or all of its nourishment, it is called phagotrophy and is distinguished from osmotrophy, which is nutrition taking place by absorption.

The history of phagocytosis represents the scientific establishment of immunology as the process is the first immune response mechanism discovered and understood as such. The earliest definitive account of cell eating was given by Swiss scientist Albert von Kölliker in 1849. In his report in Zeitschrift für Wissenschaftliche Zoologie, Kölliker described the feeding process of an amoeba-like alga, Actinophyrys sol (a heliozoan) mentioning details of how the protist engulfed and swallowed (the process now called endocytosis) a small organism, that he named infusoria (a generic name for microbes at the time).

The first demonstration of phagocytosis as a property of leucocytes, the immune cells, was from the German zoologist Ernst Haeckel. Haeckel discovered that blood cells of sea slug, Tethys, could ingest Indian ink (or indigo) particles. It was the first direct evidence of phagocytosis by immune cells. Haeckel reported his experiment in a 1862 monograph Die Radiolarien (Rhizopoda Radiaria): Eine Monographie.

Phagocytosis was noted by Canadian physician William Osler (1876), and later studied and named by Élie Metchnikoff (1880, 1883).

Phagocytosis is one main mechanisms of the innate immune defense. It is one of the first processes responding to infection, and is also one of the initiating branches of an adaptive immune response. Although most cells are capable of phagocytosis, some cell types perform it as part of their main function. These are called 'professional phagocytes.' Phagocytosis is old in evolutionary terms, being present even in invertebrates.

Neutrophils, macrophages, monocytes, dendritic cells, osteoclasts and eosinophils can be classified as professional phagocytes. The first three have the greatest role in immune response to most infections.