1. When a solution of commercial peptone is substituted for antigen in a complement-fixation test with the unheated normal serum of certain species (man, ox, sheep, horse, rabbit, white rat), a definite fixation reaction occurs both at 37° C. and at 0° C. In the ox, sheep, horse and rabbit this property of serum is partially stable at 55° C., but normal human serum and the serum of the white rat are inactive after heating at this temperature. The property is resident mainly in the carbonic-acid-insoluble globulins of the serum.
2. The same results are obtained when ethyl alcohol diluted with several volumes of normal saline solution is substituted for antigen in a complement-fixation test with normal serum.
3. Analysis of these reactions shows a close correspondence with complement-fixation by the sera of normal animals plus the Wassermann “antigen”—the Wassermann reaction of normal animals.
4. Marked complement-fixation effects are also obtained with heated normal serum of the rabbit, ox, sheep, horse plus cholesterol suspension, and particularly cholesterolised-peptone, these effects occurring in parallel with those produced by serum plus alcohol-saline, peptone solutions and the Wassermann “antigen.” The heated normal serum of the pig, white rat and guinea-pig do not exhibit these reactions, and the same applies to heated normal human serum. Unheated pig serum fails to react. Such results also elicit a close relationship between these non-specific reactions and the Wassermann reactions of normal animals.
5. The reacting property is absent from the serum (heated and unheated) of young rabbits during the first 2 to 3 weeks of life, but appears soon after this (e.g. by the 37th day) and is progressive in development. Its development in early life runs parallel to that of the natural haemolytic property of the serum for sheep's blood (due to a natural antibody-like substance). The two properties are, however, independent as illustrated by absorption tests.
6. Besides the agents referred to above as capable of fixing complement along with normal sera, other substances possess a similar property, e.g. certain alcohols, sodium oleate, tissue proteins, certain amino-acids and sodium nucleate. Commercial peptone purified by precipitation with alcohol is equally active with the original material. Cholesterolisation of these agents may yield a product whose activity is greater than that due to summation of effects.
7. Wassermann-positive and -negative human sera have been tested in the complement-fixation reaction with certain of these “pseudo-antigens,” viz. alcohol-saline, peptone, cholesterol, and cholesterolised-peptone, but a uniform parallelism has not been demonstrated between the reactions with these agents and the Wassermann effect. Some Wassermann-positive sera react also with alcohol-saline, peptone, cholesterol and cholesterolised-peptone, while sera from selected normal persons are quite inactive. A considerable proportion of Wassermann-positive sera yields definite complement-fixation with cholesterol and cholesterolised-peptone; a small proportion of Wassermann-negative sera reacts with these agents.
8. The thermolability of the serum principles acting with various “pseudoantigens” has been studied by testing unheated serum and serum heated at temperatures ranging from 46° to 60° C. Two types of thermolability curve have been demonstrated with different specimens of rabbit serum: (1) a more or less progressive weakening of the various reactions with inactivation at 60° C.; (2) inactivation of the effects with Wassermann “antigen,” alcoholsaline and cholesterol at 50–52° C., activation of the effects with the Wassermann “antigen” and cholesterol at 54–56°C. and inactivation again above 60° C.; in this case the curves for peptone and cholesterolised-peptone do not show such double inactivation. Unheated normal human serum yields reactions with the various agents (including the Wassermann “antigen”) but inactivation occurs at 50° to 54° C. whereas certain syphilitic sera yield thermolability curves somewhat similar to type (1) of rabbit serum, with inactivation at 60° C. or over.