They are the largest mono nucleated cells found in the blood stream. They are produced from hemophilic stem cells in bone marrow. Their size ranges from 10-20pm in diameter.
They circulate in blood stream for about 8 hours and migrate to tissue spaces to differentiate in to tissue specific macrophages. Phagocytic activity of monocytes increases when they differentiate into macrophages. They are dispersed throughout the body.
After differentiation some of the macrophages become fixed to reside in certain tissues and some become motile.
The fixed macrophages show amoeboid movement in the tissue to engulf foreign particles. In addition they acquire certain characters specific for their surrounding tissue. With reference to surrounding tissue, the fixed macrophages are named differently. They are as follows.
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Liver-Kuffer cells, Connective tissue -Histocytes, Bone-Osteoclasts, Kidney- Mesangial cells, Brain-microglial cells.
Blood monocytes are also heterogenous and two main subsets have been described in human blood. These are the classical CD14 ++ monocytes and the pro-inflammatory CD14 ++CD16 ++ monocytes. The latter cells are characterized by their high level production of the pro-inflammatory cytokine TNF and by their propensity to preferentially develop into potent dendritic cells.
Monocytes and macrophages play important role in immune defence, inflammation and tissue remodeling. They do so by phagocytosis, antigen processing and presentation and by cytokine production.
Certain factors such as C3b fraction of compliment system, IgG binding with surface receptors, released cytokines of TH cells; bacterial surface components etc. enhance the phagocytic activity of macrophages.
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Since they are capable of chemotaxis, they get attracted towards the pathogen or antigen antibody complex through chemotaxis.