A- Introduction:
Most immune cells are hematopoietic d`origine.
Hematopoiesis occurs in the bone marrow.
Hematopoiesis is characterized by sequential stages of differentiation defined by phenotypic expression of modification with membrane molecule and particular functional property has certain stage of maturation, proliferation and differentiation of each Lingée is control by extracellular signals in the form of contact membrane or by l`action growth factor cytokines produced by the stromal cells.
T lymphocytes and B is differentiated from hematopoietic stem cells by a common stage not yet identifies lymphoid stem cell lymphocyte these two lines is characterized by differentiation into 2 phases:
The first phase is independent of environmental stimulation.
The second phase is subjected to a mature lymphocyte activation by AG (antigens).
1- Phase central differentiation:
In the thymus for T lineage
It takes place in the bone marrow for B lineage, within these lymphoid cells central lymphoid organs undergo irreversible differentiation phenomenon resulting membrane al`expression a Ag receiver which are:
TCR (T cell receptor for T cells).
BCR (B cell receptor for B cell).
This central phase further comprises a selection process by the molecules of major histocompatibility complex HLA associated to a peptide for T lymphocytes and by the epitopes of Ag which interact with the BCR for B lymphocytes
2- Phase device differentiation:
It takes place in peripheral organs lymphocyte dependent of an activating signal induced by interaction with BCR epitope for the B and TCR peptide associated with MHC for T
B- lymphocyte lineages:
1- T cells:
Differentiation of hematopoietic precursors into T cells takes place in the thymus central phase.
Cortical thymocytes are double negative cells (CD4, CD8).
The expression of the β chain in combination with a chain pTα (substitute for the α chain is induced by the synthesis of the CD3 molecule CD = cluster of differentiation differentiation = class) that results has double positive thymocytes CD4 +, CD8 +.
These double positive cells are l`objet of positive selection + or negatively
* Positive selection:
A place in the cortex, it applies a double-thymocyte positive CD4 + CD8 + it leads to the expression of simple positive thymocyte CD4 + or CD8 + it is the interaction of TCR peptide associated with a MHC class I molecules or class II thymic epithelium cortex that l`arrêt the expression of CD4 and CD8 molecules respectively.
Class I TCD8.
Class II TCD4.
* Negative selection:
It eliminates thymocytes whose TCRs interact with high affinity with peptides is sensed by dendritic cells and macrophages in the corticomedullary junction.
The final stage of differentiation of mature T lymphocytes occurs in the secondary lymphoid organs.
Tm lymphocytes leave the thymus and pass into the bloodstream portion locates in the T-dependent areas of the secondary side lymphocyte components while the other circulates continuously between the blood and the lymph by the post capillary venule.
2- B lymphocytes:
The B cell differentiates in the bone marrow stromal cells in contact with this is the central phase.
The peripheral phase differentiation necissite this step an activating signal delivered by antigens through interaction d`une cell interdigital a lymphocyte CD4 and mature B lymphocyte this reaction takes place in the deep cortex of the lymph node or T-dependent areas of other peripheral lymphoid organs, activation of B cells that will result in differentiation centroblast then centrocyte and then plasma cell secreting d`AC and Bm cell.
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