T-Cell Development

For an effective immune response, antigens from disease-causing agents such as bacteria and viruses are recognized by the adaptive immune system. Two major components of this arm of the immune response are B and T cells, both of which have antigen receptors that respond to a vast range of antigens. These lymphocyte populations, however, develop in different tissues: for T cells, the primary lymphoid organ is the thymus. Like B cells, T cells develop from hematopoietic stem cells (HSCs) in the bone marrow, and during fetal life, in the liver. Lymphoid progenitors access circulation and migrate into the thymus. There, these cells, called thymocytes, undergo differentiation and selection steps through their interactions with thymic antigen-presenting cells. This process generates a mature repertoire of T cells that is functional—able to protect the organism from pathogens it may encounter, but also self-tolerant.