Final answer:
The two steps of the selection process in the thymus gland are positive selection, where T cells must appropriately interact with MHC molecules, and negative selection, where T cells that react to self-antigens are eliminated via apoptosis. These steps ensure the development of a properly functioning and self-tolerant T cell repertoire.
Step-by-step explanation:
The two main steps of the selection process to activate T cells in the thymus gland are positive selection and negative selection, collectively known as thymic selection. During positive selection, thymocytes develop a functional T-cell receptor (TCR) and must interact appropriately with Major Histocompatibility Complex (MHC) molecules, which is determined by thymic epithelial cells presenting MHC proteins to the untrained thymocytes. Successful binding indicates the thymocytes can correctly recognize body's own MHC, and these cells move forward in the maturation process. Conversely, negative selection occurs in both the thymus cortex and medulla, where antigen-presenting cells present self-antigens to the thymocytes. Those that bind to self-antigens signal potential for autoimmunity and are eliminated through apoptosis. This step establishes central tolerance.
These selection mechanisms ensure that only T cells with appropriate reactivity and specificity exit the thymus and enter the bloodstream as mature naïve T cells, capable of defending the body against pathogens without attacking self-tissues. It is estimated that through these processes, about 98% of thymocytes are eliminated, leaving the remaining 2% to promote a healthy immune response.