T cell development occurs primarily in the thymus, an organ located behind the sternum. It begins with precursor cells from the bone marrow that migrate to the thymus, where they undergo a series of maturation steps. These steps include the rearrangement of their T cell receptor genes, allowing them to recognize specific antigens. During development, T cells undergo selection processes to ensure they can effectively respond to pathogens while avoiding attacks on the body's own tissues. Successful T cells that pass these selection processes exit the thymus and enter the bloodstream, ready to participate in the immune response.