What is the process of T cell development?

T lymphocytes originate from hematopoietic stem cells in the bone marrow. Some of these multipotent cells differentiate into lymphoid progenitor cells that leave the bone marrow and migrate to the thymus gland through the bloodstream. Further maturation of the thymocytes or developing T-cells, takes place in the thymus. 

Within the thymus, T lymphocytes undergo a rigorous selection process. The majority of thymocytes do not survive and succumb during this selection process. Thymocytes that have receptors for self-antigens receive negative signals and are eliminated from the repository. 

Thymocytes that pass the selection process interact with self-MHC molecules and receive positive signals, ensuring their survival. The successful T-cells develop their own T cell receptor (TCR), which is specific to a particular antigen. 

The mature T-cells leave the thymus and circulate through peripheral lymphoid organs, prepared to encounter their specific antigens. On activation, these T cells grow and differentiate into effector T cells, initiating the immune response against the encountered antigen.

With age, the thymus begins to atrophy and produces fewer naive T-cells over time. This decline in T-cell diversity contributes to the increased susceptibility to infections associated with aging.