Antigen receptors are essential components of the immune system crucial in defending the body against foreign invaders. These receptors are present on the surface of B and T cells, enabling them to recognize antigens and mount an appropriate immune response.
Before encountering any antigen, lymphocytes express these receptors. On B cells, the antigen receptor is a membrane-bound antibody molecule called BCR; on T cells, it is a T cell receptor or TCR. B and T cell receptors are composed of two different polypeptide chains—heavy and light chains in the case of antibodies and alpha and beta chains in the case of TCRs.
Despite the vast number of antigen receptors required for effective immune function, human DNA only contains about 20,000 genes that code for all the proteins made by a cell. Molecular genetic studies have shown that the genes dictating the structure of each antigen receptor are not present in lymphocyte stem cells. Instead, these cells contain a few hundred genetic bits and pieces that serve as building blocks for antigen receptor genes. As each lymphocyte becomes immunocompetent, these gene segments are shuffled and combined differently. This results in the assembly of new genes expressed as the surface receptors of B and T cells and the antibodies are later released by plasma cells.
Somatic recombination is a crucial process in the immune system that allows lymphocytes to become immunocompetent and acquire the ability to recognize and resist infection by specific foreign substances. This process involves shuffling and combining gene segments in lymphocyte stem cells to generate a seemingly limitless number of unique antigen receptors. The V, D, and J gene segments are assembled to form functional antigen receptors during somatic recombination in B and T cells. This process yields a vast repertoire of immune receptors, each with a unique specificity for a particular antigen.
In summary, somatic recombination is a complex process that generates a diverse pool of antigen receptors, enabling lymphocytes to recognize and respond to a wide range of foreign invaders. Despite having only a limited number of genes, the immune system can produce a seemingly limitless number of different antigen receptors by shuffling and recombining gene segments, resulting in a highly adaptable and effective immune response.
Antigen receptors are multiprotein complexes on the B and T cell membranes that recognize specific antigens.
A B cell antigen receptor, or BCR, is a membrane-bound antibody molecule comprising a pair of heavy and light chains. In contrast, a T cell receptor or TCR primarily includes alpha and beta chains.
Both these receptors can recognize and bind to diverse antigens even before exposure. This diversity is achieved through somatic recombination.
Here, the germline genes encoding for BCR and TCR undergo recombination during the lymphocyte maturation.
During recombination, the variable or V, diversity or D, and joining or J segments of each gene are randomly selected and joined together to form a single mRNA molecule, which is later translated into a functional receptor.
These permutations produce billions of receptor chains, which are expressed as BCRs and TCRs. This enables the lymphocytes to recognize an array of antigens and mount an efficient immune response.
As a result, each lymphocyte is uniquely immunocompetent with a BCR or TCR, which remains inactive unless it encounters the corresponding antigen.
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