`T-cells are MHC restricted`. Discuss this statement.

`T-cells are MHC restricted

T-cells , which belong to a group of white blood cells known as lymphocytes , play a vital role in cell mediated immunity . They differ from other lymphocyte types , such as B-cells and Natural Killer NK cells by the presence of the T-cell receptor (TCR . T cells are called as such because their development primarily is in the thymus

T-cells are divided into two major groups . T-helper CD4 cells , the middlemen ' of the HYPERLINK "http /en .wikipedia .org /wiki /Adaptive_immune_system " \o "Adaptive immune system " adaptive immune system , secrete HYPERLINK "http

/en .wikipedia .org /wiki /Cytokine " \o "Cytokine " cytokines that holds a regulatory role in the immune response . T-cytotoxic cells CD8 kill virally infected cells and tumor cells and are also an important factor in transplant rejection

T-cells are further subdivided into Th1 and Th2 . The distinction between CD4 and CD8 occurs during their development in the thymus . This occurs only after these cells have been activated during immune activity in the peripheral lymphoid system

T-cells originate from hematopoietic stem cells in the bone marrow Hematopoietic precursors derived from hematopoietic stem cells populate the thymus and replicate to generate a large population of immature thymocytes . Thymocytes are the T-cell precursors which develop and mature in the thymus (Schwarz B A , Bhandoola A . 2006

Double-positive ' thymocytes migrate into the thymic HYPERLINK "http /en .wikipedia .org /wiki /Cortex " \o "Cortex " cortex where they are presented with host self-antigens ' complexed with Major Histocompatibility Complex HYPERLINK "http /en .wikipedia .org /wiki /Major_histocompatibility_complex " \o "Major histocompatibility complex " MHC molecules on the cortical epithelial surface . Only those thymocytes with affinity to the MHC complex receive a vital survival signal ' The remaining thymocytes die by apoptosis also known as programmed cell death and their remains are phagocytosed by macrophages . This process is called positive selection . Double-positive cells that are positively selected on corresponding MHC class II molecules become CD4 cells , and cells positively selected on matching MHC class I molecules become CD8 cells

Thymocytes that survive positive selection migrate towards the area between the thymic cortex and thymic HYPERLINK "http /en .wikipedia .org /wiki /Medulla " \o "Medulla " medulla . While in the medulla , they are again presented with self-antigen complexed with Major Histocompatibility Complex MHC molecules on HYPERLINK "http /en .wikipedia .org /wiki /Antigen_presenting_cell " \o "Antigen presenting cell " antigen-presenting cells APCs such as HYPERLINK "http /en .wikipedia .org /wiki /Dendritic_cell " \o "Dendritic cell dendritic cells and HYPERLINK "http /en .wikipedia .org /wiki /Macrophage " \o "Macrophage " macrophages HYPERLINK "http /en .wikipedia .org /wiki /Thymocytes " \o "Thymocytes Thymocytes that react with the antigen receive an HYPERLINK "http /en .wikipedia .org /wiki /Apoptosis " \o "Apoptosis " apoptosis signal that causes their suicide . A majority of all thymocytes initially produced end die during thymic selection . A small minority of the surviving cells are selected to become HYPERLINK "http /en .wikipedia .org /wiki /Regulatory_T_cell " \o "Regulatory T cell regulatory T-cells . The remaining cells will then exit the thymus as napve ' T-cells . This process is called negative selection , an important mechanism of HYPERLINK "http...