T. pallidum is antigenically complex. Infection by the bacteriainduces production of at least three types of antibodies against (a) cardiolipin antigen, (b) group-specific antigens, and (c) species-specific antigens of T. pallidum.
The cardiolipin antigen is a hapten and is chemically a diphosphatidyl glycerol. This hapten elicits the production of an antibody, known as reagin antibody, in blood of patient suf-fering from syphilis. This cardiolipin antigen is demonstrated in T. pallidum, but it is not known whether the reagin antibody is produced by cardiolipin that is present in the treponema or by the cardiolipin that is released from damaged tissues following infection by treponema.
Demonstration of the reaginic antibodies in the serum against the cardiolipin antigen forms the basis of standard non-specific tests for syphilis (e.g., VDRL, Kahn, and Wasserman).
T. pallidum group-specific antigen is protein in nature. It isfound in T. pallidum as well as in nonpathogenic cultivable treponemes, such as Reiter’s treponeme. This antigen induces the production of antibodies in the serum, which are found in patients with syphilis. Reiter’s protein complex complement fixation test, a test used in syphilis serology, employs this anti-gen for detection of serum antibodies in patients with syphilis.
Species-specific treponemal antigen is probably polysaccharide in nature. This antigen induces the development of antibodies, which can be detected in serum of the patients suffering from syphilis. The specific treponemal test, such as T. pallidum hemagglutination (TPHA) test, detects antibodies by using these antigens.