2.1 Lymphocytes:
  • Lymphocyte:   A white blood cell responsible for much of the body's immune protection.  Fewer than 1% are present in the circulating blood; the rest lie in the lymph nodes, spleen, and other lymphoid organs, where they can maximize contact with foreign antigens.
  • Lymphocytes vary from 5 to 12mm in diameter; subpopulations can be identified by unique protein groups on the cell surface called clusters of differentiation. T cells, derived from the thymus, make up approx. 75% of all lymphocytes; B cells, derived from the bone marrow, 10%. A third classification is natural killer cells (NK cells). In the blood, 20% to 40% of the white cells are lymphocytes.
    • activated lymphocyte  A lymphocyte that has been stimulated by exposure to a specific antigen or by macrophage processing so that it is capable of responding to a foreign antigen by neutralizing or eliminating it.    
    • lymphocyte activation  The use of an antigen (or mitogen in vitro) to stimulate lymphocyte metabolic activity.
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