Final answer:
Almost 95% of T cells are eliminated through apoptosis during negative selection in the thymus, which is crucial for preventing autoimmunity by removing self-reactive T cells. This is distinct from positive selection, antibody production, and MHC recognition which do not directly cause the mass apoptosis of thymocytes.
Step-by-step explanation:
Almost 95% of T cells entering the thymus die when apoptosis is triggered during negative selection. This process occurs in the thymic medulla and plays a crucial role in establishing central tolerance by eliminating self-reactive T cells that would otherwise cause autoimmunity. Self-antigens are presented to the thymocytes by professional antigen-presenting cells (APCs). Thymocytes binding to these self-antigens indicate that they would attack the body's own cells; thus, they are targeted for apoptosis and are negatively selected against. The remaining T cells are those that can bind to MHC molecules presenting foreign antigens, which is essential for the immune system to fight infections without attacking the body's own tissues.
It is during negative selection that the majority of T cell apoptosis occurs, and not during positive selection, antibody production, or MHC recognition. While positive selection also involves apoptosis, it is the process where thymocytes that can successfully bind with MHC molecules of the body are selected to continue maturation. Antibody production and MHC recognition do not directly cause the apoptosis of T cells in the thymus.