PHAGOCYTOSIS is the property possessed by many animal cells of engulfing particles by virtue of their power of amoeboid movement. Primarily this power was directed towards nutrition but in higher organisms it has been developed for different pur poses and in pathology at the present day bears a wider meaning. The particle having been englobed, one of three things may hap pen: (1) If the particle be digestible the cell secretes a digestive fluid, a food vacuole is formed, the particle is gradually dissolved by the secretion and the products absorbed into the cell sub stance. (2) If the particle be indigestible, it is retained within the cell for a time and ultimately discharged. The particle en globed may comprise almost any material, but if it is to serve as a food it must be of animal or vegetable origin. At the time of ingestion it may be dead or living. If living the organism is first killed and then digested or (3) the organism may prove resistant, multiply and finally destroy the cell, when a number of organisms are set free. This is one of the means by which, in the higher organisms, a local infection may become distributed through the organism. Digestion within a cell is fermentative; a proteolytic ferment has been prepared from the bodies of amoebae which is active in acid, neutral or especially alkaline media.
The study of phagocytosis was largely extended in the elabora tion of Metchnikoff's view of the nature of immunity. Thus, to take an instance from the sponges, minute organisms, which have penetrated the pores of the sponge are seized by the ciliated or amoeboid cells lining those spaces, and are then killed and digested. It is readily understandable that we should find such cells on the external surface of an organism or lining the alimentary tract. But in addition there are many fixed or wandering cells within the body in which phagocytic power is retained and markedly developed. They remove foreign material or debris which may occur within a tissue. For instance, as the result of an injury, inflammatory process, etc., cells and other structures of a tissue are destroyed. One of the processes preliminary to repair consists in the removal of the resulting debris, which is effected by phago cytes. A similar process is seen with red blood corpuscles which have escaped into a tissue through rupture of capillaries. Foreign particles accidentally gaining admission to a tissue are in many cases removed in a similar manner, e.g., soot particles which have passed through the respiratory surface are largely removed by phagocytes and carried to the bronchial lymphatic glands. Very
commonly living organisms effect an entrance through wound sur faces, the alimentary surface, etc., and one of the processes em ployed for their destruction and removal is that of phagocytosis. Hence the phagocytes are regarded as scavengers of the tissues.
In the above-named instances the phagocytes are chiefly wan dering cells brought to the seat of their activity by the blood. In any tissue where the process is going on phagocytes have ac cumulated in large numbers. They have been attracted to the damaged area by the chemical process of chemiotaxis. At the seat of the change chemical substances are produced (see PATH OLOGY) which act upon the phagocytes, causing them to migrate towards the source—positive chemiotaxis. The same chemical stimulus in a higher concentration may repel the cells—negative chemiotaxis. Instances of this are especially frequent in relation to micro-organisms and phagocytes.
Metchnikoff held the view, and supported it by many experi ments that immunity and susceptibility to infective disease are essentially matters of phagocytosis but this eclectic view is now obsolete though it is not denied that phagocytosis plays an im portant part in removing bacterial and other debris rendered harmless by other means (see IMMUNITY).
As regards the different varieties of phagocytes, leaving apart the cells lining the alimentary tract, a number of free cells possess amoeboid properties as well as also a number of fixed cells. In the latter category are the nerve cells, the large cells of the spleen pulp and of lymph glands, certain endothelial cells, the neuroglia cells, and perhaps certain cells of connective tissues. Those of greater phagocytic activity are the large splenic and lymph cells, the neuroglia cells and certain endothelial cells. With regard to the wandering cells some are certainly non-phagocytic, for instance the lymphocytes. The polymorphonuclear, the large hyaline and the eosinophil leucocyte are phagocytic. Metchnikoff therefore divided the phagocytes into two classes—the micro phages, comprising the polymorphonuclear and the eosinophil cell, and the macrophages, containing the large hyaline cell, the cell of the splenic pulp, the endothelial cell and the neuroglia cell.
The two principal groups of leucocytes are generally spread throughout the vertebrates. Instances of each kind are found even in the lamprey. Cells which show but small differences from the analogous cells of mammals are found in the alligator.
(T. G. B.)