<P> T cells (thymus cells) and B cells (bone marrow - or bursa - derived cells) 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, called cytotoxic 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 same pathogen is detected again; this is known as acquired immunity . </P> <P> 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 . They are named "natural killer cells" because they do not require prior activation in order to kill cells which are missing MHC class I . </P> <P> Mammalian stem cells differentiate into several kinds of blood cell within the bone marrow . This process is called haematopoiesis . All lymphocytes originate, during this process, from a common lymphoid progenitor before differentiating into their distinct lymphocyte types . The differentiation of lymphocytes follows various pathways in a hierarchical fashion as well as in a more plastic fashion . The formation of lymphocytes is known as lymphopoiesis . B cells mature into B lymphocytes in the bursa equivalent, which in humans is the GALT, which is thought to be located in the Peyer's patches of the intestine, while T cells migrate to and mature in a distinct organ, called the thymus . Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs (e.g. the spleen and lymph nodes) where they survey for invading pathogens and / or tumor cells . </P> <P> The lymphocytes involved in adaptive immunity (i.e. B and T cells) differentiate further after exposure to an antigen; they form effector and memory lymphocytes . Effector lymphocytes function to eliminate the antigen, either by releasing antibodies (in the case of B cells), cytotoxic granules (cytotoxic T cells) or by signaling to other cells of the immune system (helper T cells). Memory T cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure; they live weeks to several years, which is very long compared to other leukocytes . </P>

Where do b cells and t cells come from
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