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Introduction to the immune system
MHC class I and MHC class II molecules
B-cell activation and differentiation
Cell-mediated immunity of CD4 cells
Cell-mediated immunity of natural killer and CD8 cells
Somatic hypermutation and affinity maturation
Contracting the immune response and peripheral tolerance
B- and T-cell memory
Anergy, exhaustion, and clonal deletion
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naive T-cell activation p. 101
T cell dysfunction p. 114
T cells and p. 99
T cells and p. 411
T-cell differentiation p. 99
activation p. 101
adaptive immunity p. 97
anergy p. 108
cell surface proteins p. 108
corticosteroid effects p. 118
cytokine production p. 99, 106
cytotoxic p. 100
delayed (type IV) hypersensitivity p. 99
differentiation and maturation p. 96, 99
disorders of p. 114, 115
functions p. 99
helper p. 98
leflunomide effects p. 499
lymph nodes p. 94
major functions of p. 99
neoplasms p. 437
regulatory p. 100
sirolimus effect p. 118
spleen p. 96
thymus p. 96
untreated HIV p. 173
T cell selection in p. 100
T cell origination in p. 411
B- and T-cell disorders p. 115
Your immune system is like the military - with two main branches, the innate immune response and the adaptive immune response. The innate immune response is immediate and non-specific, meaning that although it can distinguish an invader from a human cell, it doesn’t distinguish one invader from another invader. In contrast, the adaptive immune response is highly specific for each invader, and that’s because the cells of the adaptive immune response have receptors that differentiate one pathogen from another by their unique parts - called antigens. This adaptive immune response takes days to weeks to become activated, during which time the innate immune system provides protection. The second important feature of the adaptive immune response is that it has memory, which means that once a response against an antigen is triggered, subsequent responses will be faster and stronger. This, of course, is why vaccines are so effective – once you get a vaccine containing a part of a pathogen, the next time you see that same pathogen, you will remember the previous encounter (from the vaccine) and you will kill it.
Now, the key cells of the adaptive immune response are the lymphocytes - the B and T cells - which are generated during lymphopoiesis. Lymphopoiesis has two main goals - to generate a diverse set of lymphocytes, each with a unique antigen receptor, and to get rid of lymphocytes that have receptors that are self-reactive, meaning that they could attack one’s own healthy tissue. Normally, hematopoietic stem cells within the bone marrow mature into a common lymphoid progenitor cell, which then becomes either a B-cell or a T cell. To become a B cell, it has to develop into an immature B-cell in the bone marrow and then complete its maturation in the spleen. To become a T cell, it has to migrate to the thymus and become a thymocyte, where it completes its development into a mature T cell. So, “B” for bone marrow and “T” for thymus.
T cells, also known as T lymphocytes are a type of lymphocyte that play a central role in cell-mediated immunity. The process of T-cell development begins with the migration of immature T-cell precursors from the bone marrow to the thymus gland, where they undergo a series of steps to become fully mature T-cells. In the thymus, T-cells are exposed to a diverse array of self-antigens and undergo positive and negative selection to ensure that only T-cells that recognize foreign antigens but not self-antigens are allowed to leave the thymus and enter the bloodstream.
During positive selection, T-cells that recognize self-antigens displayed by thymic stromal cells receive survival signals, allowing them to continue their development. During negative selection, T-cells that recognize self-antigens too strongly undergo apoptosis to prevent the development of autoimmunity. Once T-cells have successfully completed positive and negative selection, they leave the thymus and enter the bloodstream, where they travel to various organs and tissues to carry out their functions.
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