Immunoglobulin Are the Receptors Found on B Cells
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Immunoglobulin Are the Receptors Found on B Cells

This article is about how immunoglobulin, the B-cell receptor, promotes diversity within itself via the process of somatic recombination. The process of somatic recombination is the rearrangement of genes and is extremely precise and directly functions with the V, D, and J segments of the genes. Without this recombination event, diversity would be extremely limited.

Immunoglobulin is the receptor found on B-cells that helps it recognize antigen.  There are multiple different types of antigen; therefore, there must be different types of immunoglobulin.  This diversity is generated from within the genes that the immunoglobulin is constructed.  The immunoglobulin possesses two separate parts to its structure: the constant region and the variable region.  The antigen binds the variable region of the immunoglobulin receptor and it carries a property called specificity, which exemplifies that an immunoglobulin will bind only one specific antigen, and one only.  The genes that encode for the variable region of the receptor are segments known as the V, D, and J gene segments.  The light chain of the immunoglobulin contains only the V and J segments, whereas the heavy chain contains all three.

            To have diversity, these gene segments must be rearranged to different, varying arrangements.  The process in which this is accomplished is called gene rearrangement.  Also, it is only after gene rearrangement that the immunoglobulin gene can be transcribed and translated, thus becoming fully functional.  Before the genes become rearranged, they are known to be in the germline configuration because that is how they are present in the germ cells inherited by the organism’s parents.  This process of genetic rearrangement is known as somatic recombination.

            The benefits of somatic recombination include diversifying the specific gene arrangements of the immunoglobulin genes.  This is done by machinery that is built to “cut” and “paste” the different portions of the genes together to form multiple arrangements.  It is also capable of adding nucleotides to the sequence, thus creating more diversity.

            Somatic recombination is directed sequences of genes known as recombination signal sequences.  This RSS comprises of a 7-base sequence and a 9-base sequence.  A 12 and 23 base pair spacer separates these segments and this is true for all RSS.  This is also known as the 12-23 rule.  Once the genes have been cut, the recombination activation genes (RAG) split off the unnecessary portions of the genes so that the joining of D to J or V to DJ can occur.  

Because of somatic recombination, the diversity among immunoglobulin is highly increased.  This is only possible if the genes go through rearrangement.  They can only be arranged if they have the proper machinery to do so.  This machinery includes the RSS and the RAG proteins.  If any one of these proteins or complexes is not functional then the somatic recombination process cannot occur and thus, genetic diversity among the immunoglobulin is restricted. 

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