Phagocytosis

Phagocytosis (from Greek phago- to eat and cytos- cell), the ingestion by cells of various particles with their subsequent intracellular digestion in some cases. Phagocytosis is a phenomenon widely distributed in nature. Thus, it represents the simplest and most primitive method of feeding on solid substances in unicellular animals—amoebas, infusoria, and others. An object suitable for nutrition is phagocytized by these organisms by means of pseudopodia and undergoes digestion in special vacuoles with weakly acidic or weakly alkaline contents. In multicellular organisms, intracellular digestion also plays a very important role. Metchnikov traced phagocytosis at different stages of the animal kingdom. In the lowest Metazoa, in which the differentiation of tissues is not yet very significant, all cellular elements possess phagocytic ability, but as organization becomes more complex, derivatives of the ectoderm first lose this ability, whereas the intestinal epithelium of lower invertebrates—sponges, coelenterates, turbellarians, and some mollusks—belonging to the entoderm still retains it. The next stage of reduction is the gradual loss of the aforementioned ability by entodermal cells with the simultaneous increasingly prominent development of the ability of extracellular digestion—the secretion of digestive juices into the cavity of the intestinal tube. In some gastropods, for example, both types of digestion are present in the intestinal canal. Finally, in still more highly organized organisms and in particular in mammals, phagocytic ability is retained, with few exceptions, only in elements of mesodermal origin. Phagocytes are distinguished as mobile and immobile, microphages and macrophages. Mobile phagocytes are the cells of the blood: polymorphonuclear leukocytes, large monocytes, as well as all those elements that belong to the concept of wandering cells (see); immobile ones include: the endothelium of blood vessels and lymphatic pathways, reticular cells of bone marrow, splenic pulp, lymph nodes, Kupffer cells of the liver, histiocytes of connective tissue. Lymphocytes under normal conditions lack phagocytic ability, but when transforming into so-called polyblasts during inflammation, they acquire it. In nervous tissue, the role of phagocytes is performed by cellular elements of glia, in muscle tissue—so-called sarcoblasts, in bone tissue—osteoclasts. Microphages are represented only by leukocytes—polymorphonuclear cells, while all other phagocytes belong to macrophages. While serving a digestive function in lower animals, phagocytosis in more highly organized organisms primarily serves the purposes of resorption (dissolution of tissues during the metamorphosis of insect larvae; the reverse development of tadpoles, dissolution of cartilage; the same in various kinds of tissue degeneration, etc.). In infections, however, according to Metchnikov's theory, phagocytes capture not only dead bacteria, as asserted by supporters of the humoral direction in the doctrine of immunity, but undoubtedly also live microbes. In a number of cases, a parallelism is observed between the degree of insusceptibility of the animal and the energy of phagocytosis. At the same time, the very mechanism of phagocytosis of bacteria by leukocytes, which seemed to Metchnikov simple and exclusively cellular, turns out to be more complex, usually consisting of two phases: the preliminary opsonization of the phagocytized object (see Opsonins) and the subsequent ingestion in the proper sense of the word. Spontaneous phagocytosis (i.e., without opsonization), although observed, occurs more rarely and to a lesser degree (mainly with respect to non-virulent bacteria). Opsonins, however, enhance phagocytosis not only of bacteria, but also of particles of coal, starch grains, etc. The mechanism of opsonization, according to the research of Savchenko and Barykin, consists in a change in the physicochemical properties of the phagocytized object as a result of the adsorption of opsonins onto it, with subsequent adhesion to the leukocyte. The next phase of phagocytosis—the immersion of the phagocytized object into the body of the phagocyte—is determined by a change in surface tension in the outer layer of the phagocyte as a result of such adhesion. This mechanism of phagocytosis explains the unexpected fact for Metchnikov that not only living but also dead leukocytes can play the role of phagocytes. The degree of phagocytosis according to the Wright and Douglas technique is determined by the phagocytic index, which represents the arithmetic mean of the total number of phagocytized microbes divided by the number of phagocytes. The nature of opsonins remains insufficiently clarified for now. In addition to opsonins of normal sera, so-called bacteriotropins, or opsonins of immune sera, are also distinguished, acting in the same direction as the former. Immune opsonins, in contrast to opsonins in normal serum, are heat-stable. Nevertheless, many researchers identify normal opsonins with immune ones and explain the difference in their heat resistance to high temperatures by the greater content of opsonins in immune sera and only partial destruction of them upon heating. The question of the independent position of both types of opsonins among so-called antibodies is also unresolved: many bring normal opsonins closer to complement, and immune ones to bacteriolytic ambceptor. Metchnikov's doctrine of phagocytosis suffers from a certain one-sidedness. Undoubtedly, in immunity, essential significance belongs to factors not only of cellular but also of humoral nature, and the very act of phagocytosis, as indicated above, consists of their interaction. Metchnikov explained the bactericidal properties of sera by so-called phagolysis, i.e., the disintegration of phagocytes, during which the antibacterial substances contained in them pass into the surrounding environment, but to a certain extent the blood plasma can also be bactericidal, where mass phagolysis can no longer be spoken of.