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White blood cells called monocytes are an important part of the body’s immune system. They protect the body against microorganisms or pathogens, especially those that can cause malaria, tuberculosis, and typhoid. Moving at great speed in the bloodstream, they collect debris or remains that accumulate from infection. The immune response, accomplished by both bone marrow-derived (B) and thymus (T) lymphocytes, is activated as well by some monocytes. Lymphocytes are white blood cells that make up the lymphatic system, or the body’s second line of defense.
Monocytes count makes up 5–12% of the total white blood cell count. The biggest of the white blood cells, this type has a large nucleus that controls the activities of the cells, as well as a few granules in the cytoplasm. They are attracted to bacteria and other foreign matter, which they take in and destroy through phagocytosis. Hence, they are also referred to as phagocytes, a classification which they share with granulocytes, the most abundant of the white blood cells. Phagocytosis refers to the process by which a cell takes in large, solid materials.
Granulocytes, however, respond faster to bacterial presence than monocytes do. Nevertheless, a lot more bacteria can be destroyed by monocytes than can granulocytes. Another characteristic of monocytes is that they can break away from the blood and survive in other tissues. If this happens, a monocyte may become a macrophage, which is a phagocytic cell without granules in the cytoplasm, or a dendritic cell, which is a special cell that has extensions called dendrites. The bone marrow is the part where monocytes are produced, although it is also believed that they emerge from lymphocytes.
Macrophages function in the collection of worn-out cells and in immune response activation. To activate the body’s immune response, macrophages engulf pathogens then carry a substance that is present in the pathogens, called antigens, to T lymphocytes for identification purposes. Once antigens have been identified, B lymphocytes become activated to produce antibodies. These natural chemicals of the body neutralize the toxins of pathogens, and their continued presence ensures immunity to a number of diseases. The reaction of antibodies against antigens allows the macrophages to consume pathogens more easily, although it should be noted that not all pathogens can be digested by macrophages.
Like macrophages, dendritic cells also engulf pathogens that are captured by the cells’ numerous branches. Termed dendrites, these branches bear resemblance to the dendrites of nerve cells. Their functions, however, are not the same. The action of dendritic cells also activates T lymphocytes to perform their role in the identification of antigens.