The TPHA test is a serological test that detects antibodies produced by the body in response to a syphilis infection. It works by mixing a patient's blood serum with a solution containing particles coated with Treponema pallidum antigens. If antibodies to syphilis are present in the patient's blood, they will bind to the antigens on the particles, causing them to clump together or agglutinate. The degree of agglutination is then measured to determine the presence and concentration of syphilis antibodies.