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The general function of lymphocytes and other components of the immune system is to recognize and eliminate threats to the body. For this system to work effectively, lymphocytes must be able to distinguish between normal cells and infected cells or invasive pathogens. Each of the three types of lymphocytes has this ability. Certain lymphocytes are part of the generalized response of the innate immune system, responding quickly to threats. Others act against specific pathogens or infected cells and are part of the adaptive immune response.
Natural killer cells (NK cells) play a major role in the innate immune system. Once activated, the function of lymphocytes of this subset is to identify and destroy virally infected cells and tumor cells. Using signals detected on pathogen-infected cells that distinguish them from normal cells, the NK cells can differentiate between healthy and infected cells. NK cells then release substances that perforate the cell membrane of the abnormal cell. Once perforated, other molecules released by the NK cells enter and destroy the cell and the viruses infecting it.
The primary function of lymphocytes of the innate immune system is to deliver an immediate response to viral attack. In addition to this function, NK cells can also distinguish cells that are cancerous or have infections caused by microbes. The NK cells target and destroy these cells in the same way they eliminate cells with viral infections. Although the site and process of maturation of NK cells is not fully understood, billions are found circulating in the blood of humans at any one time.
Lymphocytes of the adaptive immune system are specific to a particular antigen — substances foreign to the body. The function of lymphocytes in this secondary line of defense is to identify pathogens and toxins that have previously been encountered. During subsequent encounters, these lymphocytes respond quickly to protect the body from infection. The adaptive immune system includes B lymphocytes, or B cells, and T lymphocytes, or T cells. Both types are produced in the bone marrow; however, B cells mature there, while T cells migrate to the thymus to mature.
B cells function to ultimately produce antibodies specific to an antigen. These antibodies are produced in large quantities, especially with repeated exposure to the antigen. T cells can be further subdivided into several types. Some direct the actions of other immune system cells, while others kill cells infected with specific pathogens. Both B cells and T cells have the capacity to remember antigens for a stronger and quicker response when the antigen is encountered in the future.