phagocytes In the 1880s, Metchnikoff, a Russian zoologist, gave the name phagocytosis (Greek phago; to eat) to the way in which a single cell organism, such as an amoeba, takes into it a solid item from its environment by extending ‘pseudopodia’ (false feet) that engulf it. He saw that some ‘white’ cells in the blood of animals ingested bacteria comparably, and that there was more of this going on in animals who had a recent infection. He proposed that these ‘phagocytes’ were the essential means of defence against invaders. In the first decade of the twentieth century, Metchnikoff was to share a Nobel prize with Ehrlich for work on immunity, whilst George Bernard Shaw took the theory on stage in The Doctor's Dilemma with the physician declaring ‘we must stimulate the phagocytes’!

Metchnikoff was right to assign importance to the phagocytes, as part of the immune system as it is now understood: they are confirmed to be unique among body cells in their function as ‘killers’. But Shaw also anticipated later knowledge: part of the immune response does indeed involve ‘stimulation’ of phagocytes, in the sense that they kill under orders from the lymphocytes that identify the invader.

The cells whose main function is phagocytosis arise initially from the bone marrow. There are two main groups: the type of white blood cells known as polymorphs (polymorphonuclear leucocytes — those with multi-lobed nuclei); and macrophages that derive from the monocytes in the blood. Cells of the monocyte–macrophage system can move in and out of the circulation by squeezing between the endothelial cells that constitute capillary walls. Substances released at sites of infection or injury attract such cells by chemotaxis: many different types of cells can produce a variety of related proteins known as cytokines and these in turn mediate a variety of inflammatory and defence mechanisms, including the attraction of both types of phagocytes. Phagocytes themselves also secrete cytokines, which influence the behaviour of other cells.

When a phagocyte ingests a potentially dangerous microorganism, it is dealt with in a set of compartments within the cell, equipped with enzymes and other substances that take hold of it, distort it, release lethal free radicals, and break down the remains.

Polymorphs mostly ingest bacteria, gathering at the site of localized infections such as pustules and abcesses; when pus forms, it is a collection of phagocytes which have died in the defence against more widespread invasion. Their numbers increase in the circulating blood during most types of more generalized bacterial infection.

Macrophages do not only ingest and destroy bacteria, viruses, inorganic foreign particles, and cellular debris. Some also act as ‘antigen presenting cells’, when peptide components of invaders are recognized on their surface and locked onto by antigen-specific lymphocytes; they provoke both the cellular and humoral components of the immune response. They also serve to pick up debris from the normal turnover of body cells, and dust particles inhaled into the lungs.

As well as the polymorphs and macrophages, there are cells that do not originate in the bone marrow, nor travel in the bloodstream, which can also act as phagocytes. Cells that line blood vessels and the blood sinuses of the liver and spleen (endothelial cells), and supportive (reticular) cells in lymphoid tissue, can all ingest foreign particles and cell debris. This dispersed group of cells gave rise to the term reticuloendothelial system.

Sheila Jennett

See also blood; immune system; infectious diseases; lymphatic system.