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‘Compare and contrast the development and differentiation of T cells and B cells, drawing out the implications for self-non-self discrimination’
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‘A fundamental requirement of the immune system is that it destroy, eliminate or inactivate all foreign viruses, bacteria and parasites without destroying self-cells or molecules’ (Lydyard 2000). This is carried out by B and T cells, which are able to distinguish what is ‘foreign’ (i.e. non self) from what is self. They are responsible for the adaptive immune response and have developed a number of different mechanisms in the central and peripheral lymphoid organs that underlie the basis for self-non-self discrimination. B and T cells are derived from the lymphoid progenitor cell and possess similar characteristics such as a large nucleus and small amount of cytoplasm, however their development processes differ in a number of important ways, the most notable being that B cells develop in the bone marrow whereas T cells migrate from the bone marrow to differentiate in the thymus. Once at the correct site both cell types undergo gene rearrangements to produce a unique antigen receptor on each cell. Figure 1: Development and differentiation of a B cell. Progenitor T cells differentiate into functional T lymphocytes under the influence of thymic stromal cells and cytokines. As seen in figure 1 the earliest recognisable B cell is referred to as a pro B cell and its specificity for a single immunoglobulin is determined by the variable region of the immunoglobulin light and heavy chains that bind to the antigen. As explained by Riott (1998) and Janeway (1999) the variable region is encoded by sets of gene segments, which are brought together by a process known as somatic recombination. The first gene rearrangement that produces the heavy-chain immunoglobulin gene segments (DH joins JH). The next stage is the differentiation of pro into pre B cells and this requires the microenvironment provided by the bone marrow stromal cells. Here we see the joining of VDJH chains causing the heavy chain to be expressed on the cell surface with a surrogate light chain producing the expression of Mu () chains. It should be noted that the V(D)J recombinase system also operates in T cells where the same core enzymes recognize the same conserved recombination signal sequences, for the T cell receptor genes (Janeway 1999). The next stage is the formation of the immature B cells. Here we see the product of light chain rearrangement replacing the surrogate light chain forming a complete IgM molecule, which is expressed on their cell surface.
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