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A lymphocyte is a type of white blood cell that is part of the immune system. The role of lymphocytes involves recognizing harmful particles, or antigens, and carrying out processes to deal with them. Different types of lymphocytes exist, known as T cells, B cells and natural killer cells, and their roles differ accordingly. T cells and natural killer cells destroy harmful cells and some T cells activate other immune cells. B cells produce antibodies, and both B and T cells create memory cells which remember threats.
Lymphocytes are produced inside bone marrow before traveling to areas of the lymphatic system such as the spleen, thymus and lymph nodes. The appearance as well as the role of lymphocytes can vary. A T or B cell is an example of what is called an agranular cell, where the gel or cytoplasm that fills the cell is clear and the nucleus is round. Natural killer cells are large granular lymphocytes, with visible granules contained in their cytoplasm and a lobed nucleus.
B and T cells have receptors on their surfaces which recognize specific antigens. These antigens could be anything that threatens the body, such as viruses, bacteria, allergic molecules or toxins. The role of lymphocytes of the natural killer cell type is not as specific and they can recognize many different types of antigen, including infected cells and some tumor cells.
T lymphocytes can be subdivided into helper and killer T cells. Helper T cells have what is probably the most important role of lymphocytes in the immune system. They activate other immune cells, including killer T cells and B cells. The main role of killer T cells is to destroy cells that have been infected by viruses. They may also attack cancer cells and cells that have been infected by bacteria.
B lymphocytes become activated when they bind to their specific antigens. Then they divide into two different types of cells, known as memory cells and plasma cells. One important role of lymphocytes is the production of antibodies, and plasma cells are responsible for this. They are able to quickly produce and release thousands of antibodies that enter the circulation, ready to attach to antigens.
Some antigens, such as viruses, can be neutralized when antibodies attach to them. A coating of antibodies can also make an antigen more attractive to a cell called a phagocyte, which may then eat and destroy it. The other type of cell produced by B lymphocytes, the memory cell, remembers antigens so that the body can respond more quickly if they attack again.