What is Neutrophils – Definition & Types, Pool & Margination

Presentation notes on Neutrophils Structure,  Size, Operation’s Mechanism in Blood of a Human Body in word /. ppt/ .pdf Format


The neutrophils have a diameter of 10-12 and are multilobed (multi lobe perhaps helps emigration of the neutrophils) WBCs containing amphophillic or neutrally stained granules. the lobes of the nucleus may be 2, 3 up to 5 but even more lobes may also be seen. Younger the cell, less will be the number of the lobes. Thus in a given blood film one may count the number of the lobes of the neutrophil.

Preponderance of younger cells (2 or 3 lobed), is called, 'shift to the left' whereas the opposite is called 'shift to the right' (Arneth count). Shift to the left may be due to infection (due to greater release of neutrophils from the RBM, so that younger cells dominate the film);shift to the right is seen in megaloblastic anemia (vit B12 ­folate deficiency). The neutrophil develops from stem cell as the cell grows it begins to acquire granules.

There are two types of granules in the neutrophil  (1) primary or lysosomal or azurophilic granules, (2) secondary granules. The primary granules begin to appear from the stage of 'promyelocyte' while the 2ndary granules appear at the stage of 'myelocyte'.Neutrophils attack the bacteria they engulf the bacteria (phagocytosis) digest them by the contents of lysosomal (primary) granules. The secondary granules, however, are not lysosomal, ie, they do not directly kill the bacteria; instead, they  are concerned with development of inflammation and other issues.

The primary (lysosomal) granules contain various chemicals, which kill the bacteria, eg, (1) hydrolase (2) mye­loperoxidase (MPO) (3) cationic proteins, (4) lysozyme and so on. They also contains 'defensins' (which are very broad spectrum antimicrobial' agents killing a wide variety of bacteria, fungus and even some virus). These agents kill the microbes by various means, eg, (i) producing oxidants (like , superoxide etc; substances like H202 destroy the bacteria), (ii) by lowering the pH within the bacteria (thus killing the bacterial enzymes) and so on.
Secondary granules, include (1) lactoferrin (2) vit B12 binding protein (3) and various sorts of receptor molecules. Major function of secondary granules is to modify inflammation.In acute bacterial infections, sometimes, bigger, sturdier, cytoplasmic granules, called, toxic granules, appear in the neutrophil. Toxic granules are immature azurophilic gran­ules.

Pool, margination and its importance

            As stated above, (i) 90% of the neutrophils are in marrow pool, (ii) 3% in the blood pool and (iii) 7% in tissue pool. The story, from 'birth to grave' of a neutrophil ('kinetics' of neutrophil) is as follows : from the stem cell, the neutrophil is born in the RBM (and constitutes marrow pool). A neutrophil spends about 10 days in marrow pool then it enters the blood pool where it sta for about 7 hrs nextly, from the blood pool, neutrophil emigrates into the tissues (to constitute the tissue pool) where it stays for about 1 to 4 days, then it (= neutrophil) dies. The dead neutrophil is removed by tissue macrophage.
Note, from the tissue pool, neutrophils do not return back to the blood.

Importance of margination:-

 As stated above, 50% of neutrophil of the blood pool stick to the vascular endothelium a phenomenon called 'margination'. (i) During inflammation, these marginated neutrophils by the help of their pseudopods, via the gap in between the endo­thelium of the capillary, move out of the capillary to reach the tissue - a phenomenon called emigration or diapede­sis. Now the emigrated neutrophil attacks the bacteria lodged in the tissue. (ii) Even normally, as stated above (kinetics of neutrophil), the neutrophils emigrate to the tissue. (iii) In the lung capillaries the phenomenon of margination is extremely important.

Mechanisms :

(i) On the neutrophils and the vascular endothelial cells, there are molecules, called, "selectins". The selectins of neutrophil have affinity for selectins of endothelial cells result is that the neutrophil comes closer to the endothelial cells;
(2) in the capillaries, the velocity of the blood is slow, therefore the neutrophils can settle down.

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