More than one-half of normal rabbits contain complement-fixing or precipitating antibodies against Wassermann antigens (the alcohol-soluble lipoids of beef, rabbit, and human hearts) by a sufficiently sensitive technique. Normal human sera tested by the same technique are uniformly negative. The intravenous injection of colloidal suspensions of beef and human heart lipoids into rabbits occasionally causes a significant increase in this normal Wassermann (antilipoid) titre. This may indicate a certain degree of antibody response to the lipoids as such; it may be due to the presence in such extracts of traces of foreign protein, which would activate the lipoid haptene into a complete antigen; or it may be a non-specific increase in a normal antibody, not due to a specific antigenic stimulus.

Confirming the results of Sachs, Klopstock, and Weil, the addition of normal foreign (human) serum to rabbit, beef, and human heart lipoids makes them antigenic for rabbits. The intravenous injection of such lipoid-serum mixtures usually causes a significant increase in the titre of the complement-fixing and precipitating antibody against tissue lipoids.

The precipitate which forms upon the addition of tissue lipoids to human syphilitic serum is by far the most efficient antigen for the production, in rabbits, of antibodies to tissue lipoids which we have as yet encountered. Rabbits injected intravenously with such a precipitate regularly develop a Wassermann titre which is many times higher than either the titre observed in human syphilis, or that induced by the injection of a normal serum-lipoid mixture. The very marked antigenic property of the precipitate as compared with that of a normal serum-lipoid mixture is considered to be due to the fact that it contains a foreign protein firmly bound to the lipoid particles, namely, the human reagin-globulin with which they have combined. This interpretation is supported by the observations (1) that heating at 100°C., which does not affect the lipoid constituent of the precipitate, destroys its antigenic power for rabbits, and (2) that a similar precipitate derived from Wassermann positive rabbit serum instead of syphilitic human serum, and therefore containing tissue lipoid in combination with homologous (rabbit) protein, is completely non-antigenic for rabbits.

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