Thymus-dependent lymphocytes. It can also be called T cells for short. Pluripotent stem cells from bone marrow (from yolk sac and liver in embryo). At present, it is believed that a part of pluripotent stem cells or pre-T cells in human bone marrow migrate to thymus, differentiate and mature under the induction of thymosin, and become immunocompetent T cells. Mature T cells are distributed to thymus-dependent areas of peripheral immune organs through blood flow, and can be recycled through lymphatic vessels, peripheral blood and tissue fluid to play a role in cellular immunity and immune regulation. The recycling of T cells is conducive to extensive contact with antigen substances entering the body, strengthening immune response and maintaining immune memory for a long time. There are many different markers on the cell membrane of T cells, mainly surface antigens and surface receptors. These surface markers are giant protein molecules bound to the cell membrane.
T cells are quite complex and heterogeneous subgroups, which are constantly updated in the body and can coexist in different developmental stages or functions. However, at present, the classification principles and nomenclature are confusing and have not been unified. T cells can be divided into several subgroups according to their different functions in immune response. It is generally believed that helper T cells (TH) have the function of assisting humoral immunity and cellular immunity. Inhibitory T cells (TS) have the function of inhibiting cellular immunity and humoral immunity. Effector T cells (TE) have the function of releasing lymphatic factor; Cytotoxic T cells (TC) have the function of killing target cells. Delayed allergic T cells (TD) have the function of participating in type ⅳ allergic reaction. Enlarged T cells (TA) can act on TH and TS, thus expanding the immune effect. Memory T cells (TM) have the function of memory-specific antigen stimulation. T cells can live in the body for months to years. Its memory cells live longer.
T cells are the main components of lymphocytes and have many biological functions, such as directly killing target cells, assisting or inhibiting B cells to produce antibodies, responding to specific antigens and mitogens, and producing cytokines. They are brave soldiers who resist disease infection and tumor formation in the body. The immune response produced by T cells is cellular immunity, which mainly has two forms: specific binding with target cells, destroying target cell membrane and directly killing target cells; The other is to release lymphatic factor, and finally expand and enhance the immune effect.
T cells, which are differentiated from lymphoid stem cells in thymus, are the most numerous and complicated lymphocyte types. According to their functions, they can be divided into three subgroups: helper T cells, inhibitory T cells and cytotoxic T cells. Their normal function is very important for human beings to resist diseases. So far, great progress has been made in the study of the evolution of T cells and its relationship with cancer. Hematopoietic stem cells, also known as pluripotent stem cells, are a group of primitive hematopoietic cells existing in hematopoietic tissues. Its biggest feature is that it can replicate and differentiate itself, usually in the quiescent period. When the body needs it, it will divide and proliferate, and some of it will differentiate into directional stem cells. After being stimulated by some hormones, it further differentiates into blood cell lines of various systems. There are two ways to further differentiate lymphoid stem cells. Some stem cells migrate to thymus, and under the influence of thymine, they proliferate and differentiate into a subset of mature lymphocytes, called T lymphocytes. The word "t" representing T cells is named after the first Latin letter of "thymus". The second cell group is influenced by hormones in organs or tissues similar to bursa of fabricius, and matures and differentiates into another subgroup of lymphocytes, called B lymphocytes. The word "b" of B cell is named after the first Latin letter of "Capsule". The bursa of fabricius is a unique structure of birds, which is located behind and above the cloaca, and its wall is covered with lymphoid tissue. It is impossible for humans and mammals to have bursa of fabricius, and its similar structure may be lymphoid tissue (collective lymph nodes, appendix, etc.). ) in bone marrow or intestine, there is also bursal effect.
T cells do not produce antibodies, but act directly. Therefore, the immune function of T cells is called "cellular immunity". B cells work by producing antibodies. Antibodies exist in body fluids, so the immune function of B cells is called "humoral immunity". Most antigen substances stimulate B cells to form antibodies; Need the help of t cells. In some cases, T cells can also inhibit B cells. If the function of inhibitory T cells is reduced due to infection, radiation, thymus dysfunction and other factors, and B cells are overactive because T cells are out of control, a large number of autoantibodies may be produced, causing various autoimmune diseases. Such as systemic lupus erythematosus, chronic active hepatitis and rheumatoid arthritis. Similarly, in some cases, B cells can also control or enhance the function of T cells. It can be seen that all kinds of immune responses in the body, whether cellular immunity or humoral immunity, constitute an extremely fine, complex and perfect defense system.
B lymphocyte
It can also be called b cells for short. Pluripotent stem cells from bone marrow. In birds, it develops in bursa of fabricius, so it is also called bursa-dependent lymphocyte for short)/bone marrow-dependent lymphocyte, which is differentiated from lymphoid stem cells in bone marrow. Compared with T lymphocytes, it is slightly larger. These lymphocytes will proliferate and differentiate into a large number of plasma cells after being stimulated by antigens. Plasma cells can synthesize and secrete antibodies and circulate in the blood. B-cell lymphoma is the most common lymphoblastic leukemia, and the research on this disease is constantly emerging. In mammals, it develops in bone marrow and other tissues with cystic structure. Also known as bone marrow-dependent lymphocytes. Stem cells or pre-B cells from bone marrow gradually differentiate into B cells with immune potential after being transplanted into bursa of fabricius or bursa-like organs of fabricius. Mature B cells migrate from peripheral blood into spleen and lymph nodes, mainly distributed in splenic nodules, splenic cords and lymph nodes, lymphatic cords and lymph nodes in submucosa of digestive tract. Stimulated by antigen, it differentiates and proliferates into plasma cells, synthesizes antibodies and plays the role of humoral immunity. The number of B cells in bone marrow and lymph nodes is more than that of T cells, but the number of B cells in blood and lymph nodes is less than that of T cells, even less in thoracic duct, and only a few participate in recycling. There are many different markers on the cell membrane of B cells, mainly surface antigens and surface receptors. These surface markers are giant protein molecules bound to the cell membrane.
B 1 cells are T cell independent cells. B2 is a T-cell dependent cell. The survival time of B cells in the body is very short, only a few days to weeks, but their memory cells can exist in the body for a long time.
K lymphocyte
Antibody-dependent lymphocytes, also known as antibody-dependent lymphocytes, are directly derived from bone marrow pluripotent stem cells. There is no antigen mark on their surface, but there is an antibody IgG receptor. In order to kill target cells, there must be corresponding antibodies to target cells. The surface antigen of the target cell binds to the corresponding antibody, and then binds to the corresponding receptor of K cell, thus triggering the killing effect of K cell. All target cells bound to IgG antibody may be killed by K cells. Therefore, it can also be said that the killing effect of K cells is nonspecific, and its recognition of target cells depends entirely on the recognition effect of specific antibodies. K cells account for about 5 ~ 10% of the total number of human peripheral blood lymphocytes, but their killing effect is very high. When there are only a few specific antibodies in the body, although they can bind antigens, they are not enough to activate the complement system and destroy the target cells, K cells can play their killing role. K cells are more in peritoneal exudate and spleen, less in lymph nodes and not in thoracic duct lymph, indicating that K cells are not involved in lymphocyte recycling. However, the killing effect of K cells plays an important role in tumor immunity, antiviral immunity, antiparasitic immunity, transplant rejection and some autoimmune diseases, and the immune response can be divided into two types: immunoprotection and immunopathology. For example, when the target cells are too large (parasites or solid tumors), phagocytes can't function, or the surface of the target cells is covered with antibodies and T cells can't get close, K cells can still function. Rejection in renal transplantation and destruction of organs or tissues involved in autoimmune diseases may all be related to K cells.
NK lymphocyte
NK cells (natural killer cells) are the third kind of lymphocytes besides T cells and B cells. The number of NK cells is relatively small, accounting for about 15% of the total number of lymphocytes in peripheral blood, and about 3%~4% in spleen. NK cells can also appear in lung, liver and intestinal mucosa, but are rare in thymus, lymph nodes and thoracic duct.
NK cells are large in size and contain cytoplasmic granules, so they are called large granular lymphocytes. NK cells can kill target cells directly, without specificity, and this natural killing activity does not require antigen sensitization in advance, nor does it require antibody participation, and there is no MHC limitation.
The target cells killed by NK cells are mainly tumor cells, virus-infected cells, larger pathogens (such as fungi and parasites), allogenic organs and tissues.
NK cell surface receptor (NKR) can recognize polysaccharide molecules expressed on the cell surface infected by virus. The killing effect of NK cells is mediated by toxic molecules released after activation, such as perforin, granzyme and TNFα.