Final answer:
The Ig that can be used to test for syphilis and mononucleosis (EBV) is the heterophile antibodies. These antibodies can be detected using immunoassay and indirect fluorescent antibody (IFA) tests.
Step-by-step explanation:
The Ig that can be used to test for syphilis and mononucleosis (EBV) is the heterophile antibodies.
In the case of infectious mononucleosis (IM), these heterophile antibodies can be detected in a patient's serum using an immunoassay test. The reaction between heterophile antibodies and animal red blood cells leads to hemagglutination, or clumping of the red blood cells, which can be visibly observed.
Similarly, in the case of syphilis, the indirect fluorescent antibody (IFA) test is used to detect antibodies in patient serum. The antibodies bind to T. pallidum bacteria, which can be visualized using a secondary antibody labeled with a fluorogen.