Why do T cells not activate in response to self antigens?

T cells do not activate in response to self antigens primarily because of the process of negative selection that occurs in the thymus. During T cell development, those T cells that express receptors that strongly bind to self antigens presented by Major Histocompatibility Complex (MHC) molecules in the thymus are typically eliminated through apoptosis. This is a critical mechanism of immune tolerance, preventing the immune system from attacking the body's own tissues, which could lead to autoimmune diseases. Therefore, T cells that are specific to self antigens are effectively killed in the thymus, ensuring that mature T cells that exit the thymus have a reduced chance of recognizing and responding to self antigens. This intrinsic quality of T cell development is essential for maintaining self-tolerance and preventing inappropriate immune responses.