Of most interest and importance in the serum diagnosis of syphilis is the question of "antigens," not only from the standpoint of efficiency and specificity, but also from the possibility of throwing more light on the nature of the reaction.
When Wassermann, Neisser and Bruck originally applied the BordetGengou phenomenon of complement-fixation to the diagnosis of syphilis, the causative organism, Treponema pallidum, had not been isolated in pure culture, so they selected the fetal liver of congenital syphilis because rich in the Treponema, for making the antigen. Securing well-marked and definite results, they concluded that the specific antigenic properties of the extract were due to the presence of the T. pallida and that the reaction was an example of specific union of antigen and antibody. Subsequent investigation indicates that this is partially true, as the antigenic principles of T. pallida are extractable in salt solution, and in some
KOLMER JA, LAUBAUGH EE, CASSELMAN AJ, WILLIAMS WW. PRACTICAL STUDIES ON THE SO-CALLED SYPHILIS "ANTIGENS," WITH SPECIAL REFERENCE TO CHOLESTERINIZED EXTRACTS. Arch Intern Med (Chic). 1913;XII(6):660-677. doi:10.1001/archinte.1913.00070060052005