T cell and B cell Maturation

T cells originate from haematopoietic stem cells in the bone marrow and undergo positive and negative selection in the Thymus (Bursa) to mature. 

T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity

All T cells originate from haematopoietic stem cells in the bone marrow. The maturation of most T cell proceeds along 2 different developmental pathways which generate functionally distinct C4, C18 (sub-population that exhibit class II and Class I).

T cell maturation takes place in thymus. Progenitor T cells from early sites of haematopoiesis begin to migrate the thymus. T cell maturation involves rearrangements of germ line TCR genes and expression of various membrane markers. In Thymus developing T cells (Thymocytes) proliferate and differentiate along developmental pathways that generate functionally distinct subpopulation of mature T cells.

About 98% of Thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent T cells.

Aside from being main source of all T cell, it is where T cells diversify and then are shaped into an effective memory T cells repatoir by  Selection process.

Positive selection:

It permits survival of only those T cells whose TCR are capable of recognizing  self MHC molecule.

It is thus responsible for  the creation of a self MHC restricted repatoir of T cells.

Negative selection:

Eliminates T cells that react too strongly with self MHC. It important factor in generating a primary t cell repatoir ie self tolerence.

Immature B cells are produced in the Bone marrow and migrate to secondary lymphoid tissues where some develop into mature B cells.

B cell development occurs through several stages, each stage representing a change in the genome content at the antibody loci.When the B cell fails in any step of the maturation process, it will die by a mechanism called apoptosis, or specifically, clonal deletion. When the B cell receptor, on the surface of the cell matches the detected antigens present in the body, the B cell proliferates and secretes a free form of those receptors (antibodies) with identical binding sites as the ones on the original cell surface. After activation, the cell proliferates and B memory cells would form to recognize the same antigen.

The immature B cells whose B-cell receptors (BCRs) bind too strongly to self antigens will not be allowed to mature. If B cells are found to be highly reactive to self, three mechanisms can occur. Each B cell has a unique receptor protein (referred to as the B cell receptor (BCR)) on its surface that will bind to one particular antigen. The BCR is a membrane-bound immunoglobulin.

The B cell may either become one of these cell types (Plasma cell or  memory cell) directly or it may undergo an intermediate differentiation step, the germinal center reaction, where the B cell will hypermutate the variable region of its immunoglobulin gene ("somatic hypermutation") and possibly undergo class switching. Other functions for B cells include antigen presentation, cytokine production, and lymphoid tissue organization.

-Dixy