Phagocytosis

Phagocytosis (

is a form of endocytosis wherein large particles are enveloped by the cell membrane of a (usually larger) cell and internalized to form a phagosome, or "food vacuole."

In animals, phagocytosis is performed by specialized cells called phagocytes, which serve to remove foreign bodies and thus fight infection. In vertebrates, these include larger macrophages and smaller granulocytes, types of blood cells. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytosed. Virulent bacteria may need to be coated in antibodies before they can be consumed. Certain pathogenic bacteria, such as those of leprosy and tuberculosis, once internalized through phagocytosis, are resistant to killing by the phagocytes that have ingested them. Anything that impedes or prevents the action of phagocyctes is termed antiphagocytic.

Received wisdom dictates that macrophages and neutrophils represent professional phagocytes, but this merely reflects the greater amount of research performed on these cell types. 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.

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.

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

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 amoebae, 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.