Question

1. Why do naïve T cells migrate preferentially to lymphoid organs and differentiated effector T cells (which have been activated by antigen) migrate preferentially to tissues that are sites of infection? other (bystander) T cells on exposure to an antigen?

Answer 1

T cells: T cells (also called T lymphocytes) are one of the major components of the adaptive immune system. Their duties include directly killing infected host cells, activating other immune cells, producing cytokines and regulating the immune response.

Production:T cells originate from haematopoietic stem cells that are produced in the bone marrow. Some of these multipotent cells will becomes progenitor cells that leave the bone marrow and travel to the thymus via the blood. In the thymus these cells mature. T cells are named after their thymus-dependent development. Each T cell will develop its own T cell receptor (TCR) that is specific for a particular antigen. T cells that survive thymic selection will mature and leave the thymus. They will circulate through the peripheral lymphoid organs, each ready to encounter a specific antigen and be activated. Once activated, the T cell will proliferate and differentiate into an effector T cell.

A naive T cell  is a T cell that has differentiated in bone marrow, and successfully undergone all the processes of central selection in the thymus. A naive T cell is considered immature and, unlike activated or memory T cells, has not encountered its cognate antigen within the periphery.Naive T cells are continually generated in the thymus, where each cell undergoes DNA rearrangement to generate a unique T-cell receptor and where each cell is screened against autoreactivity and for an affinity (“restriction”) .

Naive T cells preferentially migrate to  lymphoid organs, due to antigen encounter happening there and   mature after antigen acquisition in nonlymphoid tissues and migrate to the lymph node.Naive T cells enter lymph nodes from the blood via specialized vascular regions called high endothelial venules (HEV).If a naïve cell does not bind any of the MHC/peptide complexes it encounters, it exits through the efferent lymphatics, which ultimately drain into the thoracic duct and then back into the blood.Each naive T cell recirculates from blood through a lymph node and back to blood every 12 to 24 hours. Because only about 1 in 100,000 naive T cells is likely to respond to any given antigen, this rapid recirculation increases the chances that a T cell will encounter appropriate antigen.

.The differentiated Effector T cell are a group of  cells that includes several T cell types that actively respond to a stimulus, such as co-stimulation. It includes CD4+, CD8+, Treg cells.The migration of antigen-specific T cells to nonlymphoid tissues is for the elimination of foreign antigens from the body.These effector cells then leave the secondary lymphoid organs and use newly acquired trafficking receptors to extravasate at sites of inflammation.

Other T cells on exposure to antigens respond to the microbial antigens. Once activated, some of the T cells then migrate to the site of infection, where they help other phagocytic cells, mainly macrophages to destroy the microbes.