What is a Reagin?

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  • Written By: C. Mitchell
  • Edited By: John Allen
  • Last Modified Date: 08 October 2019
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A reagin is any of a family of antibodies that usually appear as an immunoglobulin in blood, but sometimes also occur in human skin serums. Reagin antibodies are often an indicator of certain conditions and diseases. The antibodies usually form as a result of other cell denigrations in the body, including those caused by syphilis. In other cases, reagins form in reaction to certain histamines, which can trigger allergic reactions in some people. Nothing about the presence of reagins in a person’s body is necessarily bad. How the reagins react with the body’s cells and why they were formed in the first place is what makes it significant.

Not all people have reagin antibodies, and even those that do may not have them forever. Reagins form in response to a specific condition or cellular presence in the body. Doctors generally regard the presence of reagins as a signal of something bigger, and as a reason to conduct further tests.

The Rapid Plasma Reagin test (RPR), for example, is a common preliminary syphilis test. It works by testing a patient’s blood for reagin antibodies. When a person is infected with syphilis, the syphilis virus, through the bacteria Treponema Pallidum, destroys many of the body’s healthy cells. The parts of those cells that are not consumed by the bacteria are left as carnage in the blood stream, and the body will in most cases create a reagin antibody to combat them.


The RPR is a well-respected syphilis screening test, and it can also be used to determine how effectively syphilis treatments are working in patients with established infections. The RPR test alone cannot offer a definitive diagnosis, however. Any presence of reagin in the blood, whatever its cause, will lead to a positive RPR result. Other conditions that can cause a positive RPR include malaria, lupus, lyme disease, and HIV. Because syphilis-specific tests are usually more invasive than a simple blood draw, the RPR is generally the first step in any suspected syphilis case.

Reagin in the blood might also indicate the presence of certain allergic tendencies. When some of antibodies attach to cell membranes in the body, they can trigger the release of histamines, which in turn cause allergic reactions. The reactions can range from skin rashes to hay fever, pollen allergies, and, in extreme cases, asthma.

Reagins are generally specific to the individual in which they form, and are not present in the majority of people. They do not pass the placental barrier, so cannot pass from mother to child. They do not typically survive blood transfusions. Exposure to heat or oxygen usually destroys them.


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