A<unk>  is an immune cell present in the<unk> us, before it undergoes transformation into a T cell.<unk> s are produced as stem cells in the bone marrow and reach the<unk> us via the blood.<unk> opoiesis describes the process which turns<unk> s into mature T cells according to either negative or positive selection. This selection process is vitally important in shaping the population of<unk> s into a peripheral pool of T cells that are able to respond to foreign pathogens but remain tolerant towards the body's own antigens. Positive selection selects cells which are able to bind MHC class I or II molecules with at least a weak affinity. This eliminates (by a process called "death by neglect") those T cells which would be non-functional due to an inability to bind MHC. Negative selection destroys<unk> s with a high affinity for self peptides or MHC. This eliminates cells which would direct immune responses towards self-proteins in the periphery. Negative selection is not 100% effective, and some autoreactive T cells escape and are released into the circulation. Additional mechanisms of peripheral tolerance exist to silence these cells, but if these fail, autoimmunity may arise.
Thymocyte
Monocyte
Lymphoblast
Podocyte
Agranulocyte
Prolymphocyte
Basophil
Plasma cell