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An idiotype is a set of amino acids that are unique to an antibody and that determine its specificity — in other words, which antigens it will bind to. The study of idiotypes is important for immunologists who need to understand how the body produces and uses antibodies. It is possible to harness idiotypes in vaccine production to trigger a patient to start producing appropriate antibodies and provide protection against certain antigens. It is possible to sequence these amino acids in a lab environment.
These structures are often located at or near the antigen binding site, on a part of the antibody known as the complementarity determining region (CDR) or hypervariable region. A single antibody can have multiple idiotypes. The idiotype is extremely specific — so much so that humans can sometimes react to their own idiotypes and certainly will react to those introduced from other people or species. When this is done deliberately with a vaccine, it can stimulate the immune system into the production of antibodies to resist infectious disease.
This is one method of classifying antibodies. At a slightly broader level, antibodies can be classified by allotype, genetic variations in antibody structure that can reflect genetic heritage. Closely related people tend to have similar allotypes but can react against allotypes produced in the bodies of people who are not related. This can be an issue with procedures, such as blood transfusions, in which antibody matching is critical to prevent adverse reactions.
Even more broadly, antibodies can be classified by isotype, a characteristic seen in all members of the same species. Immunoglobulin M, for example, is a human isotype, a protein that is found in every person except those who have a genetic disorder that impairs immune function. The progressive classification of antibodies can help researchers as they study antibodies and their functions.
The idiotype is highly unique; only antibodies that have identical specificity will have the same idiotype, but antibodies that have different specificities can have the same isotypes and allotypes. Researchers can use lab testing to identify particular idiotypes in a sample and sequence them to learn more about how they work. It is possible to use laboratory animals to produce human idiotypes of a particular specificity for use in the research and development of new vaccines. Idiotype variations also can explain why people might have antibodies to one strain of disease but not others, because they cannot lock on to antigens with which they are not designed to be paired, as seen in cases of influenza.