Phagocyte

A phagocyte is a cell that ingests and destroys foreign matter such as microorganisms or debris by a process known as phagocytosis.

Types of Phagocytes
There are two main categories of phagocytes:


 * macrophages (and monocytes)
 * microphages, such as polymorphonuclear leukocytes (primarily neutrophils)

Functions of Phagocytes
Phagocytes are extremely useful as an initial immune system response to infection. If the skin is broken, neutrophils are the first type of phagocyte to migrate to sites of injury and fight bacteria by releasing cytotoxic granules and by phagocytosis. Phagocytosis is an active process in wound healing.

Phagocytes contain many lysosomes that enable them to digest foreign material. Phagocytes engulf pathogens, debris, dead or dying cells and extracellular matrix. After engulfment into a phagosome, a lysosome which is filled with digestive enzymes (proteases and oxygen radicals) will fuse with it to form the phagolysosome. The two compartments become one phagolysosome, in which the phagocytosed material is digested.

In the case of pathogen phagocytosis, professional antigen-presenting cells (i.e.: dendritic cells and macrophages) will present on their surfaces small peptides that resulted from the digestion, bound to MHC class II molecules. Helper T cells (CD4+) later recognize these antigens presented through MHC class II complemented with a second signal and they will further supplement the cell mediated immune response.

In addition to engulfing and digesting foreign particles, phagocytes can induce apoptosis of normal and tumor cells, produce cationic proteins, complement components and clotting factors, arachidonic acid metabolites, prostaglandins, leukotrienes, thromboxanes, cytokines, proteases and hydrolases, reactive oxygen and nitrogen intermediates.

Pathogen Resistance to phagocytes
Many pathogens can delay or prevent the creation of the phagolysosome such as Mycobacterium tuberculosis, Salmonella typhi and Legionella. Others, such as the parasites of the genus Leishmania, are capable of resisting or circumventing being digested in the phagolysosome.

One function of T-helper cells is to activate phagocytes to digest intracellular pathogens.

Etymology
The word phagocyte etymologically means "cell that eats", originating from the Greek words phagein, meaning 'eat', and kytos, meaning 'hollow'.