"Reagin" is an outdated term that was used to describe a type of antibody found in the blood serum of some individuals, particularly those who have had certain infectious diseases or who have allergies. These antibodies were known as "reaginic antibodies" and were characterized by their ability to cause a positive reaction in a test called the "Reagin test" or "Wassermann test."

The Reagin test was developed in the early 20th century and was used as a diagnostic tool for syphilis, a sexually transmitted infection caused by the bacterium Treponema pallidum. The test involved mixing a patient's serum with a suspension of cardiolipin, lecithin, and cholesterol - components derived from heart tissue. If reaginic antibodies were present in the patient's serum, they would bind to the cardiolipin component and form a complex that could be detected through a series of chemical reactions.

However, it was later discovered that reaginic antibodies were not specific to syphilis and could be found in individuals with other infectious diseases or allergies. As a result, the term "reagin" fell out of favor, and the test is no longer used as a diagnostic tool for syphilis. Instead, more specific and accurate tests, such as the Venereal Disease Research Laboratory (VDRL) test and the Treponema pallidum particle agglutination (TP-PA) assay, are now used to diagnose syphilis.