Lymphocyte

A lymphocyte is a kind of white blood cell in the vertebrate immune system,^[1] specifically, a landmark of the adaptive immune system.

Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells (NK cells). Small lymphocytes consist of T cells and B cells.

T cells and B cells[edit source | editbeta]

Main articles: T cell and B cell

T cells (thymus cells) and B cells (bursa-derived cells^[2]) are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity, whereas B cells are primarily responsible for humoral immunity (relating to antibodies). The function of T cells and B cells is to recognize specific “non-self” antigens, during a process known as antigen presentation. Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogens or pathogen-infected cells. B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses. In response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune response, while other T cells, calledcytotoxic T cells, produce toxic granules that contain powerful enzymes which induce the death of pathogen-infected cells. Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal these memory cells will “remember” each specific pathogen encountered, and are able to mount a strong and rapid response if the pathogen is detected again.

Natural killer cells[edit source | editbeta]

Main article: Natural killer cell

NK cells are a part of the innate immune system and play a major role in defending the host from both tumors and virally infected cells. NK cells distinguish infected cells and tumors from normal and uninfected cells by recognizing changes of a surface molecule called MHC (major histocompatibility complex) class I. NK cells are activated in response to a family of cytokines called interferons. Activated NK cells release cytotoxic (cell-killing) granules which then destroy the altered cells.^[3] They were named "natural killer cells" because of the initial notion that they do not require prior activation in order to kill cells which are missing MHC class I.