Mastering T Cell Knowledge

T Cells: Production, Maturation, and Classes

• Humoral immunity fights pathogens in extracellular spaces, while cellular immunity targets and eliminates intracellular pathogens through the actions of T cells.
• T cells are formed from multipotent hematopoietic stem cells in the bone marrow and differentiate first into lymphoid stem cells that become small, immature lymphocytes.
• Immature T lymphocytes enter the bloodstream and travel to the thymus for the final steps of maturation, where they are referred to as thymocytes.
• The maturation of thymocytes within the thymus can be divided into three critical steps of positive and negative selection, collectively referred to as thymic selection.
• Thymic selection eliminates thymocytes with defective T-cell receptors, those that do not interact appropriately with MHC molecules, and self-reacting thymocytes by apoptosis.
• Peripheral tolerance involves mechanisms of anergy and inhibition of self-reactive T cells by regulatory T cells to prevent autoimmune disease.
• T cells can be categorized into three classes: helper T cells, regulatory T cells, and cytotoxic T cells, differentiated based on their expression of certain surface molecules, their mode of activation, and their functional roles in adaptive immunity.
• Helper T cells and regulatory T cells are characterized by the expression of CD4 on their surface, while cytotoxic T cells are characterized by the expression of CD8.
• Helper T cells and regulatory T cells can only be activated by APCs presenting antigens associated with MHC II, while cytotoxic T cells recognize antigens presented in association with MHC I.
• Helper T cells serve as central orchestrators that help activate and direct functions of humoral and cellular immunity, enhance the pathogen-killing functions of macrophages and NK cells.
• Regulatory T cells' primary role is to prevent undesirable and potentially damaging immune responses, protect against autoimmune disorders.
• Cytotoxic T cells are the primary effector cells for cellular immunity, recognize and target cells that have been infected by intracellular pathogens, destroying infected cells along with the pathogens inside.