The origin of LT

The origin of LT We have a stem cell at the level of the MOH which will give T precursors which will leave the MOH to go to the level of the thymus which is an odd organ in the middle position in the superior tarot of the mediastinum and it is in this thymus that the T Lymphocytes will carry out their maturation and education. T lymphocytes must acquire the capacity to recognize antigens in association with MHC class II molecules, therefore they must have a certain affinity for class II molecule antigen complexes. The second point is that T lymphocytes do not must not recognize self-antigens. In the thymus there are epithelial cells which will express the self antigen and if the T lymphocytes recognize the self antigens there is an autoimmune disease and a destruction of certain tissue by our own LT therefore it is necessary that the LT which recognize the self antigen are destroyed and made to the level in the thymus are not delivered survival signals to these T lymphocytes so at this time they die of apoptosis and are phagocytosed by thymus macrophages, therefore maturation and education. The LTs that come out of the thymus are CD3 and they are either CD4 or CD8. After an antigenic contact the LT will transform into an immunoblast and nothing distinguishes morphologically the type B or T immunoblast. The T immunoblast induces the immune response and can evolve and give rise to a memory T lymphocyte. There are 3 categories of T Lymphocytes: ➔ Cytotoxic LT: With B lymphocytes represent effector cells because they can directly destroy a cell. ➔ Auxiliary LT and suppressor: are regulatory cells of the immune system. Cytotoxic LT are essentially represented by CD8 + lymphocytes, they will recognize a foreign antigen in association with MHC class I molecules, for example a tumor cell which will synthesize abnormal antigens but given that it is a cell which derives from a normal cell it will continue to express the molecules class I which are expressed by the majority of cells in our body. At this time, the cytotoxic T lymphocyte can carry out its function and destroy the tumor cells. If, for example, the tumor cell no longer expresses class I molecules, at that moment there is an immune escape, the cytotoxic T lymphocyte is no longer able to go destroy tumor cells because there is no longer any expression of the antigen in association with class I molecules and this is one of the ways for a tumor to continue to grow. We can have CD4 which exert a cytotoxic action but it is not direct but goes if exercised through the action of cytokines. Helper T lymphocytes are mainly T4, they will induce and amplify the immune response via interleukins (example:the B lymphocyte which is stimulated by the macrophage thatI will synthesize interleukin 2 which will self-stimulate it by autocrine). Suppressor T lymphocytes are represented by CD8 and they will represent the loop negative regulation to avoid the phenomenon getting out of control. Zero lymphocytes:NK (neither T nor B) to be active requires interleukin 2 and these LK in the presence of Il2 will give LAK cells for lymphokine activated killer, LAK cell which will be able to destroy targets which are devoid of class I molecules of the MHC mais which are covered with antibodies and we will speak of an antibody dependent cytotoxicity therefore a cytotoxicity which depends on antibodies.

-Lymphoid tissue:spatial organization of these lymphoid cells ➔ Main role :protect the body against foreign substances and against adherent cells such as tumor cells, this lymphoid tissue is formed by organs such as the thymus, the spleen, the lymph nodes, the tonsils, the hematopoietic bone marrow and we also include the lymphocytes of the circulating blood, those of the lymph, the clusters of lymphocytes found in connective tissue, for example those found in level of the digestive or respiratory mucosa.