Monocyte

Monocytes are 12-20 µm large cells belonging to the leukocytes. Thus, they are the largest cells of the peripheral blood. Monocytes are found in the blood of all mammals and birds.

Monocytes are derived from pluripotent bone marrow cells, and differentiate into tissue-derived macrophages via various intermediate stages. In vitro, they can differentiate into any cell of the monocyte-macrophage system.

Monocytes have a more or less lobulated sometimes only kidney-shaped nucleus with a loose variably dense chromatin scaffold that appears mesh-like, interspersed with chromatin clumps. The nucleus occupies about half of the cell. The cytoplasm is gray to blue-gray, often vacuolated, and fine azurophilic granules are frequently found, around some of which a halo is discernible. Lysosomes are found in the cytoplasm, which contain peroxidase as well as acid hydrolases and play an important role in phagocytosis.

Receptors for complement factors and Fc receptors for IgG, among others, are found on the cell membrane; these are important in the attachment of substances to be phagocytosed.

After 1-2 days of circulation in the peripheral blood, differentiation into tissue macrophages takes place. In healthy individuals, up to about 8% monocytes are found in the peripheral blood.

Monocytes can be identified cytochemically (see "α-Naphthyl acetate esterase in blood smear").

Normal value (EDTA blood): 90 - 600/µl (1% - 12%).

Pathologically elevated: mononucleosis, syphilis, malaria, bacterial endocarditism Tbc, brucellosis, malignant NHL, monocyte leukemia, Crohn's disease, ulcerative colitis, sarcoidosis, convalescent infections.