Phagocytosis

Overview
Phagocytosis is the cellular process of engulfing solid particles by the cell membrane to form an internal phagosome, or "food vacuole." The phagosome is usually delivered to the lysosome, an organelle involved in the breakdown of cellular components, which fuses with the phagosome. The contents are subsequently degraded and either released extracellularly via exocytosis, or released intracellularly to undergo further processing.

Phagocytosis is involved in the acquisition of nutrients for some cells, and in the immune system it is a major mechanism used to remove pathogens and cell debris. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytosed.

Phagocytosis is a specific form of endocytosis involving the vesicular internalization of solid particles, such as bacteria, and is therefore distinct from other forms of endocytosis such as pinocytosis, the vesicular internalization of various liquids.

Etymology
Phagocytosis is named from Greek roots: phagein - to eat, kytos - cell, and -osis - a suffix indicating a process; thus it can be translated literally as "the cellular process of eating." It should be noted that the process is only 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.

Phagocytic cells
Many protists are able to phagocytose particles; however, in animals phagocytosis is performed by specialized cells called phagocytes, which are able to remove foreign bodies and thus fight infection. In humans and many other animals, phagocytes include macrophages, monocytes, dendritic cells, and granulocytes. The term professional phagocytes can be used to describe both macrophages and neutrophils, as these cells are considered to have phagocytosis as their primary function.

A great body of evidence continues to mount showing that resident, neighbouring cells in a tissue will phagocytize their apoptotic neighbours, thus maintaining tissue homeostasis. This clearance can, depending on the location, facilitate greater clearance than that achieved by resident macrophages.

Innate immunity
In humans, the most important facet of phagocytosis is its control of inflammation. Depending on the phagocytosed particle, phagocytosis can induce inflammation or, as is the case with apoptotic cells, induce resolution of inflammation. Phagocytosis is also involved in immune tolerance, which prevents inflammation against normal components of the body.

Adaptive immunity
For the activation of the adaptive immune response, phagocytosis is a necessary activation step. A T helper cell must be presented foreign particles bound to the major histocomptability complex class II (MHC II) receptor to become activated.

Nutrition
In many protists, phagocytosis is used as a means of feeding, providing part or all of their nourishment. This is called phagotrophic nutrition, as distinguished from osmotrophic nutrition, which takes place by absorption. In some, such as amoeba, phagocytosis takes place by surrounding the target object with pseudopods, as in animal phagocytes. In other protozoa, for instance, ciliates, there is a specialized groove or chamber in the cell where phagocytosis takes place, called the cytostome or mouth. The resulting phagosome may be merged with lysosomes containing digestive enzymes, forming a phagolysosome. The food particles will then be digested, and the released nutrients diffused or transported into the cytosol to use in other metabolic processes.

Resistance to phagocytosis
A substance or process which impedes or prevents the action of phagocytes is termed antiphagocytic (for example, Yersinia pestis, the bacterium causing the plague). Additionally, some bacteria may need to be opsonized before they are recognised as particles to be phagocytosed.

Certain intracellular pathogens, such the bacterial agents of leprosy and tuberculosis, are resistant to lysosomal degradation once internalised via phagocytosis.