The so-called syphilitic antibodies can be removed from a serum by means of absorption with barium sulphate. The removal is due either to an adsorption or a mechanical absorption. The activity of the syphilitic antibodies is thereby unimpaired. The readiness with which the absorption is accomplished with barium sulphate varies considerably with different syphilitic sera. That barium sulphate exerts the same absorbing effect upon non-syphilitic serum components is made evident by the interfering property which the latter manifest in the absorption experiment of the syphilitic antibodies. The selective removal of the serum components, other than the syphilitic antibodies, by means of barium sulphate absorption is, therefore, impossible.
On the other hand, a partial removal of these components, with but little removal of the syphilitic antibodies, may be effected when the content of a given serum is poor in syphilitic antibodies and comparatively rich in the indifferent serum components. But this is impossible if the conditions are reversed. The main reasons why some negative syphilitic sera may be so modified by the barium sulphate treatment as to give positive reactions, are explained below, but these apply only to those methods in which inactivated serum is employed. The inactivation reduces the antibody content to about one-fourth to one-fifth of the original. When the serum is very rich in antibodies, this does not affect the result of the fixation test. But when the amount of the antibodies is small, the process of inactivation creates conditions quite unexpected. It may produce such a condition that a given amount of the serum contains, after inactivation, only one or two antibody units, while the other serum components remain undiminished. Here one must not lose sight of the vital fact that these apparently indifferent serum constituents are not at all indifferent in the fixation processes. They may possess affinities which are similar to those of complement for the fixing combination of syphilitic serum and antigen. Speaking quantitatively, one unit of the syphilitic antibodies plus antigen will fix 0.1 cubic centimeter of guinea pig complement, but this unit can also be saturated and blocked by nearly the same amount of the seemingly indifferent serum component of the serum to be tested. Moreover, the regular amount of inactivated serum used in the Wassermann system is 0.2 cubic centimeter, a quantity sufficient to saturate two units of the fixing combination. Fortunately, this self-saturation of the syphilitic antibody-antigen combination by the other serum components is not constant in occurrence, owing to the wide variations of the fixability of the serum components of man. Here the benefit of Wechselmann's procedure becomes obvious. By removing a surplus of the fixable indifferent serum components by means of barium sulphate, the serum is made to react positively, or more strongly than before the treatment with barium sulphate. This masking of the positive reaction through the self-saturation is liable to occur in any system in which inactivated serum is recommended.
Another equally important factor in masking the positive reaction in a serum in which the antibodies are poor, is the presence in considerable amount of natural antisheep amboceptor in human serum. It is a plain and simple fact that an excess of hemolytic amboceptor renders a positive reaction feeble or completely negative. As we have shown in our present investigation, a hemolytic amboceptor can be removed from the serum by means of absorption with barium sulphate. Thus it is easy to understand why Wechselmann found that barium sulphate absorption improves the reaction in the original Wassermann system.
The treatment of syphilitic serum with this salt can have a twofold benefit in the case of the original method of Wassermann; namely, the removal of certain interfering serum components and the removal or diminishing of the natural antisheep amboceptor present in the syphilitic serum.
In the method of Noguchi, there is no necessity for applying the barium sulphate absorption. Noguchi recommends the use of unheated serum, hence the absolute amount of the serum employed is only one-half of the absolute amount of complement. Eventually an old serum may be anticomplementary and need inactivation, but if the result is doubtful in this instance, a fresh serum from the same patient may be secured and subjected to reëxamination. In this method there is no danger of introducing an amount of hemolytic amboceptor which is both unknown and uncontrollable, for the reason that human serum is usually devoid of isolysin (antihuman hemolytic amboceptor), and, if the latter is present, it never reaches the strength which shows any effect upon the hemolytic system employed.
We conclude, therefore, that the barium sulphate absorption is to be recommended for the original method of Wassermann under certain conditions, but that it is unnecessary for the antihuman hemolytic system of Noguchi.