Epitope = peptide sequence/motif of the target protein/molecule/structure recognized by the antibody
Paratope = sequence within the CDR recognizing the epitope.
Dear Dr. Saroj Kumar Khan
Paratope:
A paratope is the region of antibody that recognizes and binds to the epitope of an antigen. The paratope region is present in the Fv region of the antibody and consists of 5-10 amino acids. Paratope are produced by the complementary binding of light and heavy chains that create a three-dimensional structure.
The functional immunoglobulin genes within B lymphocytes, the cells that differentiate to antibody-producing plasma cells, are formed by gene rearrangements and recombination. These events give rise to the production of different variable domains in each B lymphocyte. Once a functional gene has been constructed, no other rearrangements are allowed to take place within this cell. This dictates that one particular cell will produce antibodies with an identical antigen-combining site, and is known as allelic exclusion.
When a B lymphocyte is first stimulated by antigen it produces IgM. As the immune response develops, the class of antibody changes. However, the immunoglobulin produced will have the same variable domain and therefore bind to the same antigen. All that is altered, or switched, is the heavy-chain constant region. Thus the progeny of a single B cell will produce different immunoglobulin isotypes as the response to a particular antigen develops, but each will have the same paratope.
Epitope:
An epitope is the part of an antigen that the host’s immune system recognizes, eliciting the immune response to an invading pathogen. It specifically binds to the corresponding antigen receptor on the immune cell (such as a B cell) and binding only occurs if the structures are complementary.
As far as epitope is concerned, there are three types of epitopes:1) conformational, 2) linear, and 3) discontinuous. This classification is based upon their structure and their interaction with the antibody’s paratope.
Conformational epitopes are formed through the interaction of amino acid residues which are disconnected from each other.
Linear epitopes are determined not just by their primary structure (sequence of amino acids) but also by other residues present. More distant amino acid residues of the antigen as well as those which flank the primary structure affect the linear epitope’s three-dimensional conformation.
Discontinuous epitopes consist of parts of the protein which are brought together by protein folding rather than being close to each other in the structure. This class of epitope can contain both conformational and linear parts.
Some antigens like polysaccharides, usually have many epitopes, but all of the same specificity. This is because polysaccharides may be composed of hundreds of sugars with branching sugar side chains, but usually contain only one or two different sugars. As a result, most shapes along the polysaccharide are the same.
On the other hand, antigens such as proteins usually have many epitopes of different specificities. This is because proteins are usually hundreds of amino acids long and are composed of 20 different amino acids. Certain amino acids are able to interact with other amino acids in the protein chain and this causes the protein to fold over upon itself and assume a complex three-dimensional shape. As a result, there are many different shapes on the protein. That is why proteins are more immunogenic than polysaccharides as they are chemically more complex.
For instance, if you consider a microbe, such as a single bacterium, it has many different proteins (and polysaccharides) on its surface that collectively form its various structures, and each different protein may have many different epitopes. Therefore, immune responses are directed against many different epitopes of many different antigens of the same microbe. If you consider even simple viruses, it possesses many different epitopes.
Regards,
Malcolm Nobre
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