The fixing property of a specific precipitate and of syphilitic serum in the presence of certain antigenic lipoids, can be removed by adding certain non-complementary proteins of blood serum or hen's egg.

This disappearance of the complementary activity in the syphilis reaction, as well as in the true Bordet-Gengou reaction, is a phenomenon which incidentally accompanies the fixation of certain serum constituents, some of which possess a complementary activity. The presence or absence of the complementary property in these protein components does not influence fixation. Whether the disappearance of the complementary activity during the phenomenon of so-called fixation is due to a mechanical precipitation of the molecules through absorption or whether it is due to a physico-chemical alteration of the active molecules, is unknown. It is more probable that a chemical interaction takes place in the case of the syphilis reaction. Certain sera, for example, those derived from man and goat, show a low fixability.

It is interesting to note that the fixability is gradually diminished when these sera and egg-white are heated to a temperature above 56° C., and totally disappears at 85° C. The coagulation of proteins with absolute alcohol or by boiling, destroys their interfering property.

The fact that the fixation is not selectively directed towards complement, has a very important meaning for exact serology. The one-sided accuracy as to the complementary unity is no longer sufficient for quantitative work. Both the complementary and the volumetric unity of a serum serving as the source of complement should be taken into consideration. Besides, the fixability of the sera of various species of animals must also be considered.

From these facts a formula may be derived for deciding the degree of suitableness of a serum.

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X is the degree of suitableness; K, the species constant for the fixability; P, the complementary activity; and V, the volume of serum. It will be seen that the suitableness is proportional to the fixability constant and the complementary unity, and inversely proportional to the volume of serum employed.

As to what species yields the largest value for X, we refer the reader to our studies published elsewhere.

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