What is Monocyte – Normal Count & Function in Blood

Presentation Notes on Monocytes Cell Low High Range, Roles in Human Blood, Development, Origin & Life Span for Medical & Biology Students in word / .doc / .ppt format 


In the peripheral blood, monocytes, accounting for about 2-8% of the leukocytes are present. These monocytes then leave the blood and enter the tissues where they are known as tissue macrophages. (phagos = eat; macro = big; that is, "big eaters"). Therefore, monocytes of blood and tissue macraphages are often considered together and known as "reticuloendothelial system", RES. However, most modem authors think that the term RES, a name first coined by Aschoff and Landau of Germany, long ago, is unsatisfactory, and various alternative names have been proposed. But no alternative name is universally agreed. One such alternative name "mononuclear phagocytes" (hence, mononuclear phagocyte system, MPS) is popular among American authors. Further, till recent past, monocytes or rather, mononuclear phagocytes, were viewed, only or at least mainly, as a phagocyte, engulfing bacteria — cellular debris etc.Today we know, that in addition to phagocytosis, monocytes also secrete many chemicals and these chemicals have important roles to play in the development of inflammation — cardiovascular shock, in septicemia —bacterial infection induced fever and many other things.

A third recent addition to our knowledge is that the mononuclear phagocyte cells play a strong role in immunity.Monocytes are the largest circulating leukocyte (diameter 12-20 μ). Nucleus is not lobed but the nucleus is usually ovoid and is often indented and sometimes indented deeply to present a horse shoe shaped appearence; though the monocyte is a nongranulocyte, the cytoplasm contains many granules which are very fine.

Kinetics, origin, development, life span

Monocytes are born in the RBM, from the CFU, GM →becomes a blast cell called monoblast  → eventually become monocyte and enters peripheral blood.In the blood it stays for about 48 or even 72 hrs . Then it enters, the tissue where it is now known 'as tissue macrophage. In the tissues, the macrophage probably remains alive for about 3 months.


As stated above the mononuclear phagocyte has 3 major sets of functions: (1) phagocytosis, (2) secretion and (3) role in lymphocyte mediated immunity. It also (4) has a role in tissue repair.

(1) As a phagocyte, the mononuclear phagocyte of blood (that is, monocyte) is very powerful. It emigrates into the tissues following acute bacterial invasion, but about, 24 hrs after the neutrophil emigration (hence, monocytes are called "second line of defense") and engulfs the bacteria →subsequently, usually (but not invariably) digests the bacteria. The process of digesting the bacteria is more or less similar to that of the neutrophil, that is, (i) producing lethal killing agents (eg, H202, (ii) lowering the pH etc but monocytes do not have the MPO; monocytes also engulf bacteria in the blood.
The tissue macrophages, can engulf a wide variety of substances — cell debris/ dead RBC/bacteria/foreign bodies and so on.                                    

 (2) Monocytes as well as tissue macrophages can secrete quite a large variety of chemicals, most notably:
 (i) IL-I, (ii) TNFa, (ill) binding proteins like transferrin, (iv) lysozyme, (v) proteases, (vi) acid hydrolase and so on.

Major effects of IL-1 include (a) it acts as a WBC growth factor in RBM ; (b) acts as a fever producing agent (pyrogen) and so on. Tumor necrosis factor α (TNFα) plays a leading role in the causation of shock due to grn -ve bacteria induced septicemia. TNFα has also many other functions, including destruction of the invading bacteria.
Monocyte-tissue macrophages also play a key role in the development of lymphocyte mediated immunity. Thus, the monocytes act as APC, antigen presenting cells to the lymphocytes. Monocyte macrophagee secretory products also play a key role in healing and repair.

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