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A lymphocyte is a type of white blood cell in the human immune system. All lymphocytes derive from a common progenitor, which itself originates in the stem cells in the bone marrow.

Types of lymphocytes

There are three types of lymphocytes. The T cells and B cells are especially important in the adaptive immune response, in which antigen-specific lymphocytes proliferate and eliminate pathogens. The natural killer cells (NK cells) are large, granular cytotoxic lymphocytes important in the innate immune response.

T cells

T cells mature in the thymus (hence "T") and are chiefly responsible for cell-mediated immunity. There are three types of T cells:

  • Cytotoxic T cells (CD8+) destroy infected cells which express specific antigens that they recognize. Clinical CD8 counts are now used with clinical CD4 counts as significant surrogate markers of HIV infection (see CD4 helper cells below).
  • Helper T cells (CD4+) facilitate communication between other lymphocytes in the immune system by the secretion of cytokines. One of the major claims of the HIV/AIDS hypothesis is that HIV destroys CD4+ cells; indeed, sufficiently small clinical counts of these CD4 helper cells is now considered sufficient by the CDC to warrant a diagnosis of AIDS.
  • Regulatory T cells (CD4+CD25+)(also known as suppressor T cells) suppress activation of the immune system and maintain immune system homeostasis. Failure of this regulatory T cell functioning may result in autoimmune diseases.

B cells

B cells, named for the bursa of Fabricius in which they mature in bird species, (hence "B") are thought to mature in the bone marrow in humans. B cells are primarily responsible for humoral immunity (relating to antibodies). 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. There are two types of B cells:

  • Plasma B cells secrete antibodies which bind to antigens, making them easier targets for phagocytes.
  • Memory B cells are formed specific to the antigen(s) encountered during the primary immune response. Since memory B cells live long, they are able to respond quickly upon second exposure to the same antigens.

A critical difference between B cells and T cells is how each cell "sees" an antigen. B cells recognize their cognate antigen in its native form. In contrast, T cells recognize their cognate antigen in a processed form - as a peptide in the context of an MHC molecule.

NK cells

Natural killer cells (NK cells) attack infected cells and possible cancerous cells by inserting proteins called perforins into the plasma membrane of the cell they are attacking.

Viewing lymphocytes

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.

Lymphocyte counts

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, 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.


This page uses content from the lymphocyte article on Wikipedia, captured on 10 Feb 2006. The list of authors can be seen in the page history. As with the AIDS Wiki, the text of Wikipedia is available under the GNU Free Documentation License.
This page uses content from the B_cell article on Wikipedia, captured on 11 Feb 2006. The list of authors can be seen in the page history. As with the AIDS Wiki, the text of Wikipedia is available under the GNU Free Documentation License.