Leukocytes

LEUKOCYTES (from Greek leukos-white and kytos-cell), white, or colorless bodies, one of the types of blood formed elements alongside erythrocytes and thrombocytes. The term "leukocyte" is used in two senses: 1) to denote all varieties of white blood cells; 2) to denote a certain category of white granular or granulocytes (formerly and monocytes), opposed to lymphocytes; this meaning, however, is not generally accepted and is retained mainly in pathology and clinic. L. constitute an essential component of blood and lymph not only of vertebrates but also of invertebrates (amoebocytes). Their general appearance is the same everywhere: these are colorless spherical cells capable of amoeboid movement, with a nucleus, and with homogeneous or granular protoplasm; but in details the structure and functions of L. differ both in the same animal and in different species. A rational classification of varieties of L. has not yet been worked out even for the best-studied object—the blood of man, and the comparison of forms established for man with forms of other mammals and vertebrates cannot be carried out completely. The first classifiers (Wharton Jones, 1846; Max Schultze, 1865) distinguished besides lymphoid bodies, considered elements of lymph, thin-granular and coarse-granular forms. The foundations of the modern classification were laid by Ehrlich (1877-98), who preliminarily studied the staining reactions of different types of grains in man and other animals and established 5 types of granularity, designating them by Greek letters (a-acidophilic or eosinophilic, §-initially indulinophilic, later amphophilic, i.e. staining with both acidic and basic dyes, u-basophilic, of the mast cell type, b-thin basophilic, e-neutrophilic). In human blood Ehrlich established 5 types of L.: 1) lymphocytes without grains, 2) mononuclears and transitional forms (monocytes, splenocytes, lympholeukocytes, leukoid lymphocytes), to which he erroneously attributed granularity §, 3) neutrophils, 4) eosinophils (acidophils, oxophils), 5) basophils, or mast L. [see separate table (st. 463-464), figure 3]. By origin all these types are divided into 2 groups: lymphoid (from lymph. glands and spleen), to which lymphocytes and mononuclears belong, and myeloid (from bone marrow)—all the rest. This classification has been retained up to the present time in clinical examination of blood, for which it was essentially intended, but in theoretical respect it raises a number of objections, and the principles laid in its basis are hardly likely to be retained. In non-granular forms a special azureophilic granularity was described (Michaelis, Wolff); neutrophilic granularity, as was shown by many authors, can be stained with both acidic and basic dyes, i.e. bears an amphophilic character; but the most important—Ehrlich's staining does not allow the homologization of leukocytes of different animals. The basophils of man and guinea pig have a completely different character of grains; leukocytes, by form and quantity corresponding to human neutrophils, in the rabbit and guinea pig are distinctly acidophilic (pseudo-eosinophils), and in the rat, mouse, cat they contain no stainable grains at all. On this basis Weidenreich and Maximov strike out the term "neutrophil" and replace it with another: "special granular L." On the other hand, the belonging of monocytes to the lymphoid group is disputed in recent times—they are connected with endothelium and reticuloendothelium. Thus, a rational classification of L. is a matter of the future. Structure of L. In the protoplasm of L., besides the indicated protein inclusions, which are extremely diverse in form, iodophilic grains (glycogen), fat droplets, which are stained with Sudan III, vacuoles, and quite special inclusions (Kulovsky bodies in monocytes) are encountered. Flemming and M. Heidenhain established the presence in L. of a cellular center in the form of a centrosome surrounded by a light field free of grains; it is usually placed in an indentation of the nucleus; in amphibians one can observe fibers going from the centrosome to the periphery of the cell (organic rays of M. Heidenhain). Fuchsinophilic granularity, known to hematologists under the name "granularity of Altman-Schridde," represents, as is now proved, mitochondria; they could be stained with Janus green in a fresh preparation. Finally, in the protoplasm of all types of leukocytes, with an appropriate technique, the Golgi apparatus is detected, located in the same place as the cellular center. The nucleus of L. is characterized by a large amount of chromatin in the form of angular clods; sometimes it is stained throughout (compact nuclei of lymphocytes); in other forms the chromatin network is rarer, and then nucleoli are visible. The shape of the nucleus is extremely diverse and characteristic for different types of L. The nucleus can be round (lymphocytes), oval or bean-shaped (monocytes), horseshoe-shaped ("transitional forms"), in the form of a curved rod (Stab nuclei of neutrophils) and finally lobed, in the form of separate segments (from 2 to 5), connected by thin bridges (granular leukocytes); such forms were formerly considered multinuclear. In pathological cases (infectious diseases, intoxication, neoplasms, etc.) various types of toxic granularity are detected in the protoplasm of L., and the nuclei and their chromatin undergo changes in relation to form, location, and staining. In some animals (rats, amphibians) ring-shaped nuclei, simple or segmented, are encountered. Physiology of L. The basic manifestation of the vital activity of L. is their ability to amoeboid movement (see), inherent in all types of L. In the current of blood L. are immobile, but encountering any solid object (e.g. the wall of a vessel), they adhere to it, flatten out (thigmotaxis), begin to emit pseudopodia and crawl from place to place. Closely connected with this is their ability to pass through the wall of vessels into the surrounding connective tissue (emigration) and back from tissue to vessel (immigration), as well as to exit from the organism (through the epithelium of the digestive tract). The L. that have exited from the vessels form wandering cells (see) of connective tissue; it was formerly thought that their movement is determined by guiding irritations of a chemical nature (chemotaxis—see), which conditions their accumulation in certain foci of irritation; however, at present the phenomenon of emigration of L. and the movement of L. in tissue are explained physically and chemically (see Inflammation, Infiltration). The second property of L. (except lymphocytes) is their ability to phagocytosis, i.e. the absorption of small particles (dyes, coal, bacteria, fragments, cells, erythrocytes, etc.), and, if they are of organic nature, their digestion (intracellular digestion). According to Metchnikoff's theory, L. plays the main role in the struggle of the organism with bacteria (see Immunity), in the restructuring of the organism during development and in phenomena of aging. The third property of L. is their secretory capacity, which Ehrlich connected with granularity, placing L. thus in the category of glandular elements. The humoral theory of immunity attributes to L. the ability to release alexines, which explains their bactericidal properties. Besides, L. produce and secrete various enzymes; granular L. (and partly monocytes) give a positive reaction for oxidase and peroxidase; they also contain a proteolytic enzyme causing the dissolution of tissues (which is observed in abscesses and in processes of organ restructuring); after death they undergo autolysis (see); in lymphocytes the presence of lipase is proved. To what extent these inclusions play a role in all these processes remains unclear, and Ehrlich's doctrine is subjected to strong criticism in this point (Weidenreich). (See also Hematopoiesis, Blood).

V. Karpov