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A lymphocyte is a type of white blood cell involved in the vertebrate immune system. There are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes. The large granular lymphocytes are more commonly known as the natural killer cells (NK cells). The small lymphocytes are the T cells and B cells. Lymphocytes play an important and integral part of the body's defenses.
Types of lymphocyte The three major types of lymphocyte are the natural killer (NK) cells, T cells and B cells. NK cells are a part of cell-mediated immunity and act during the innate immune response. They can attack host cells that display a foreign (e.g. viral) peptide on particular cell surface proteins known as MHC class I molecules. Once they determine a cell is infected, the NK cells release cell killing (cytotoxic) granules that will destroy the infected cell. NK cells do not require prior activation in order to perform their cytotoxic effect upon target cells. Like NK cells, the T cells are chiefly responsible for cell-mediated immunity whereas B cells are primarily responsible for humoral immunity (relating to antibodies). T cells are named such because these lymphocytes mature in the thymus; B cells (named for the bursa of Fabricius in which they mature in bird species) are thought to mature in the bone marrow in humans. T and B lymphocytes differ from NK cells in that they are the principal cells involved in the adaptive immune system. These are cell types that retain a memory of a previous infection so that they can respond to the same infectious agent quickly upon reinfection. In the presence of an antigen, B cells can become much more metabolically active and differentiate into plasma cells, which secrete large quantities of antibodies. T cells, after they see an antigen, will also become highly activated and will secrete specific proteins, such as cytokines and cytotoxic granules, depending on their subtype/function. Lymphocyte development Mammalian stem cells differentiate into several kinds of blood cell within the bone marrow. This process is called haematopoiesis (hematopoiesis = U.S. spelling). All lymphocytes originate, during this process, from a common lymphoid progenitor before differentiating into their distinct lymphocyte types. The formation of lymphocytes is known as lymphopoiesis. B cells remain in the bone marrow to mature, while T cells migrate to and mature in a distinct organ, called the thymus. Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs (e.g the spleen and lymph nodes) where they survey for invading pathogens and/or tumour cells. The lymphocytes involved in adaptive immunity (i.e. B and T cells) differentiate further after exposure to an antigen; they form effector and memory lymphocytes. Effector lymphocytes function to eliminate the antigen, either by releasing antibodies (in the case of B cells), cytotoxic granules (cytotoxic T cells) or by signaling to other cells of the immune system (helper T cells). Memory cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure. Recognizing lymphocytes
Lymphocytes and disease The human immunodeficiency virus (HIV) hijacks and destroys T cells (specifically, the CD4+ subgroup of T lymphocytes). Without the key defense the these T cells provide, the body is susceptible to opportunistic diseases that otherwise would not kill healthy people. The level of HIV progression is typically determined by measuring the percentage of CD4+ T cells in the patient's blood. The effects of other viruses or lymphocyte disorders can also be estimated by counting the numbers of lymphocytes. This lymphocyte count is part of a peripheral complete blood cell count and is expressed as percentage of lymphocytes to total white blood cells counted. An increase in lymphocytes is usually a sign of a viral infection (in some rare cases, leukemias are found through an abnormally raised lymphocyte count in an otherwise normal person). A general increase in the number of lymphocytes is known as lymphocytosis whereas a decrease is lymphocytopenia. See also | ||||||||||
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