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Lymphocytes

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Lymphocytes
T Lymphocyte

T Lymphocyte

Image Credit: NIAID

Lymphocytes

Lymphocytes are a type of white blood cell generated by the immune system to defend the body against cancerous cells, pathogens, and foreign matter. Lymphocytes circulate in blood and lymph fluid and are found in body tissues including the spleen, thymus, bone marrow, lymph nodes, tonsils, and liver. Lymphocytes provide a means for immunity against antigens. This is accomplished through two types of immune responses: humoral immunity and cell mediated immunity. Humoral immunity focuses on identifying antigens prior to cell infection, while cell mediated immunity focuses on the active destruction of infected or cancerous cells.

Types of Lymphocytes

There are three main types of lymphocytes: T cells, B cells, and natural killer cells. Two of these types of lymphocytes are critical for specific immune responses. They are B lymphocytes (B cells) and T lymphocytes (T cells).

B cells develop from bone marrow stem cells in adults. When B cells become activated due to the presence of a particular antigen, they create antibodies that are specific to that specific antigen. Antibodies are specialized proteins that travel thorough the bloodstream and are found in bodily fluids. Antibodies are critical to humoral immunity as this type of immunity relies on the circulation of antibodies in bodily fluids and blood serum to identify and counteract antigens.

T cells develop from liver or bone marrow stem cells that mature in the thymus. These cells are responsible for cell mediated immunity. T cells contain proteins called T-cell receptors that populate the cell membrane. These receptors are capable of recognizing various types of antigens. There are three major classes of T cells that play specific roles in the destruction of antigens. They are cytotoxic T cells, helper T cells, and regulatory T cells.
  • Cytotoxic T cells directly terminate cells containing antigens by binding to them and lysing or causing them to burst open.
     
  • Helper T cells precipitate the production of antibodies by B cells and also produce substances that activate other T cells.
     
  • Regulatory T cells (also called suppressor T cells) suppress the response of B cells and other T cells to antigens.
Natural killer (NK) cells function similarly to cytotoxic T cells, but they are not T cells. Unlike T cells, the NK cell's response to an antigen is nonspecific. They do not have T-cell receptors or trigger antibody production, but they are capable of distinguishing infected or cancerous cells from normal cells. NK cells travel through the body and can attach to any cell that they come in contact with. Receptors on the surface of the natural killer cell interact with proteins on the captured cell. If a cell triggers more of the NK cell's activator receptors, the killing mechanism will be turned on. If the cell triggers more inhibitor receptors, the NK cell will identify it as normal and leave the cell alone. NK cells contain granules with chemicals inside that, when released, break-down the cell membrane of diseased or tumor cells. This ultimately causes the target cell to burst. NK cells can also induce infected cells to undergo apoptosis (programmed cell death).

Memory Cells

During the initial course of responding to antigens such as bacteria and viruses, some T and B lymphocytes become cells known as memory cells. These cells enable the immune system to recognize antigens that the body has previously encountered. Memory cells direct a secondary immune response in which immune cells such as cytotoxic T cells and antibodies are produced more quickly and for a longer period of time than during the primary response. Memory cells are stored in the lymph nodes and spleen and can remain for the life of an individual. If enough memory cells are produced while encountering an infection, these cells can provide life-long immunity against certain diseases such as mumps and measles.
 
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