Leukocytes also known as white blood cells (WBCs) are cells which play a defensive roll in the body against injury and infection. They migrate towards tissues where they are needed and become functional performing various activities. WBCs can be divided into 2 groups: Polymorphonuclear granulocytes and mononuclear agranulocytes. Both types are spherical while suspended in blood plasma but when they invade tissues after leaving the blood vessels they become amoeboid and motile. The sizes referred to in the diagrams are as observed in blood smears (where cells are spread out, here they can appear slightly larger than they are when in circulation in the blood plasma). Granulocytes have 2 types of granules:
• Specific granules – these have specific functions and bind to several stains (neutral, basic or acidic)
• Azurophilic granules – these are specialized lysosomes which stain darkly and are present at some level in all forms of leukocytes.
When WBCs phagocytise microorganisms many azurophilic granule proteins work together to kill and digest the microorganisms. Bacterial proteins include:
• Myeloperoxidase – which generates hypochlorite and other reactive agents which are toxic to bacteria
• Defensins – which bind and produce holes in cell membranes of microorganisms
• Lysozymes – which dissolve the components of the walls of bacterial cells
Granulocytes contain nuclei with two or more lobes including – neutrophils, eosinophils and basophils. These terminally differentiated cells only have a life span of a few days. They have poorly developed Golgi complexes and rough ER and they contain only a few mitochondria. Leukocytes depend largely on glycolysis for their low energy requirements this allows them to function in tissue which has little oxygen present e.g. inflamed areas. These cells usually die by apoptosis in the connective tissue. Billions of neutrophils die apoptotic deaths every day in the adult human. The cellular debris that results is removed by macrophages.
Agranulocytes have no specific granules but do have azurophilic granules, they have a round or slightly indented nucleus and include lymphocytes and monocytes. All leukocytes are key in the defence against microorganisms as well as in repairing injured or damaged tissue. There are about 6000 – 10000 leukocytes per microliter of blood however this can vary depending on age, sex and physiological conditions.
Monocytes are bone-marrow derived agranulocytes with a diameter of between 12 and 20 µm. A monocytes nucleus is large and off centre, it may be oval, kidney or distinctly u-shaped. Monocytes have chromatin which is less condensed (and stains lighter) than that of lymphocytes. The cytoplasm of these leukocytes is basophilic and contains very small azurophilic granules (these granules are distributed throughout the cytoplasm and they give off a bluey-grey colour in stained smears). Nucleoli, a small quantity of rough ER, free polyribosomes and many small mitochondria are present in monocytes but these are only visible under electron microscopy, many microvilli and pinocytotic vesicles can be found at the cells surface. Golgi apparatus are also present these are involved in the formation of lysosomes. Circulating monocytes are the precursor cells of the mononuclear phagocyte system. After monocytes cross the walls of the postcapillary venules they differentiate into microglia in the CNS, macrophages in connective tissues and osteoclasts in the bone and so on.
Lymphocytes constitute a separate family of leukocytes that can be subdivided into functional groups’ distinguished (immunocytochemically) by their distinctive surface molecules. T-lymphocytes, B-lymphocytes and natural killer cells are of the best known of the lymphocytes, which have diverse functional roles including defending against invading microorganisms, foreign or...
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