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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. In the presence of an Antigen , B cells can become much more Metabolically active and differentiate into Plasma Cell s, which secrete large quantities of Antibodies . Microscopically, in a Wright's Stained Peripheral Blood Smear , a normal lymphocyte has a large, dark-staining nucleus with little to no Basophilic cytoplasm. In normal situations, the coarse, dense nucleus of a lymphocyte is approximately the size of a red blood cell (about 7 micrometres in diameter). Some lymphocytes show a clear perinuclear zone (or halo) around the nucleus or could exhibit a small clear zone to one side of the nucleus. It is impossible to distinguish between T cells and B cells in a peripheral blood smear. Normally, Flow Cytometry testing is used for specific lymphocyte population counts. When one must specifically determine the percentage of lymphocytes that produce a particular secretion (say, a specific antibody or Cytokine ), the ELISPOT or Secretion Assay techniques can be used instead. The Human Immunodeficiency Virus (HIV) hijacks and destroys T cells (specifically, CD4 + lymphocytes). Without this key defense, the body is susceptible to Opportunistic Disease s that otherwise would not kill healthy people. A 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, Leukemia s 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 . The formation of lymphocytes is known as Lymphopoiesis . SEE ALSO Granulocytes Normal blood contains from 5000 to 9000 White Blood Cells (WBC’s) per cubic millimeter and 70 to 75% of them are Granulocytes . These cells are formed and mature in the bone marrow and are continually being released into the blood. They make up 20 to 25% of the WBC’s. The granulocytes have multilobular nuclei and are 9 to 12 mm in diameter. (Out of the Texas Woman's University Microbiology Text Book.) link title ]
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