Neutrophils are the most common granulocyte. They have segmented nuclei, typically with 2 to 5 lobes connected together by thin strands of chromatin which can be difficult to see; the cell may thus appear to have multiple nuclei. The nuclear chromatin is condensed into coarse clumps. Small numbers of immature neutrophils or band form neutrophils may be seen in a blood smear. These are incompletely segmented and often have a 'C-shaped' nucleus.
The cytoplasm of neutrophils contains three types of granule.
Primary granules are non-specific and contain lysosomal enzymes, defensins, and some lysozyme. The granules are similar to lysosomes. They stain aviolet colour when prepared with Wright's stain which is commonly used in studying the blood. The enzymes produce hydrogen peroxide which is a powerful anti-bacterial agent.
Secondary granules are specific to neutrophils and stain light pink ('neutral stain'). They contain collagenase, to help the cell move through connective tissue, and lactoferrin, which is toxic to bacteria and fungi.
Tertiary granules have only recently been recognised. It is thought that they produce proteins which help the neutrophils to stick to other cells and hence aid the process of phagocytosis.
Once in the area of infection neutrophils respond to chemicals (called chemotaxins which are released by bacteria and dead tissue cells) and move towards the area of highest concentration. Here they begin the process of phagocytosis in which they engulf the offending cells and destroy them with their powerful enzymes. Because this process consumes so much energy the neutrophils glycogen reserves are soon depleted and they die soon after phagocytosis. When the cells die their contents are released and the remnants of their enzymes cause liquefaction of closely adjacent tissue. This results in an accumulation of dead neutrophils, tissue fluid and abnormal materials known as pus.
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