Lymphocyte

A lymphocyte is a type of white blood cell in the vertebrate immune system. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic adaptive immunity), and B cells (for humoral, antibody-driven adaptive immunity). They are the main type of cell found in lymph, which prompted the name “lymphocyte”. Lymphocytes make up between 18% and 42% of circulating leukocytes.
Any of the mononuclear nonphagocytic leukocytes found in the blood, lymph, and lymphoid tissues; they comprise the body’s immunologically competent cells and their precursors. They are divided on the basis of ontogeny and function into two classes, B and T lymphocytes, responsible for humoral and cellular immunity, respectively. Most are small lymphocytes 7–10 μm in diameter with a round or slightly indented heterochromatic nucleus that almost fills the entire cell and a thin rim of basophilic cytoplasm that contains few granules. When activated by contact with antigen, small lymphocytes begin macromolecular synthesis, the cytoplasm enlarges until the cells are 10–30 μm in diameter, and the nucleus becomes less completely heterochromatic; they are then referred to as large lymphocytes or lymphoblasts. These cells then proliferate and differentiate into B and T memory cells and into the various effector cell types, B cells into plasma cells and T cells into helper, cytotoxic, and suppressor cells.