Leukocytes

white blood cells, colorless blood cells of animals and man.

Leukocytes and erythrocytes, or red blood cells, have a common origin, in both phylogeny and ontogeny. Most of the blood cells in invertebrates are amoebocytes, which are similar to leukocytes. In vertebrates, including man, the blood cells are clearly divided into leukocytes and erythrocytes. There are 5,000–8,000 leukocytes, on the average, per cu mm of blood of a healthy human adult.

A distinction is made between agranular leukocytes (agranulocytes), the cytoplasm of which contains no permanent inclusions, and granular leukocytes (granulocytes), which contain cytoplasmatic granules. The agranulocytes include the lymphocytes, a group of cells with heterogeneous functions that participate mainly in immunity reactions, and the monocytes, which are capable of phagocytizing large foreign particles (including the remains of dead cells) and which make up part of the reticuloendothelial system. The agranulocytes, a source of substances that stimulate cell reproduction and phagocytosis, play an important role in the processes of inflammation, healing, and regeneration. The granulocytes include the eosinophils, containing granules that stain with acid dyes; the basophils, which contain heparin and histamine and whose granules stain with basic dyes; and the neutrophils, which are rich in hydrolytic enzymes, function as lysosomes, and whose granules do not usually stain.

Neutrophils are capable of moving and of ingesting small foreign particles, including microbes. Releasing hydrolytic enzymes, they can dissolve, or lyse, necrotic tissue (for example, with inflammation or regeneration). The function of the eosinophils and basophils has not been elucidated conclusively. The number of leukocytes and the percentage ratio of their varieties (the leukocyte formula) vary with species, age, and physiological condition, and with the presence of disease. Determinations of the number of leukocytes and of the leukocyte formula are used extensively in medical and veterinary practice for diagnostic purposes (see Table 1).

Table 1. Leukocyte formula in healthy human adult
	Maximum range (percent)
Basophils..............................	0.5–1
Eosinophils.............................	2–4
Neutrophils
Myelocytes............................	-
Immature.............................	0–1
Rod-nuclear...........................	3–5
Segmented-nuclear.......................	51–67
Lymphocytes............................	21–35
Monocytes.............................	4–8

REFERENCES

Mechnikov, I. I. Lektsii o sravnitel’noi patologii vospaleniia. Moscow, 1947.
Khrushchov, G. K. Rol’leikotsitov krovi v vosstanovitel’nykh protsessakh v tkaniakh. Moscow-Leningrad, 1945.
Kassirskii, I. A., and G. A. Alekseev. Klinicheskaia gematologiia, 4th ed. Moscow, 1970.

G. A. ALEKSEEV and N. G. KHRUSHCHOV