The failure of union between antigen and precipitin when present in the same serum

It has been frequently observed that the serum of rabbits, immunized with foreign protein, may, at certain times, contain not only precipitin, but also unaltered antigen remnants. Such sera not only precipitate the antigen, but also give precipitates when mixed with other antisera prepared with the same antigen (Linosier et Lemoine, Eisenberg, Michaelis, and Fleischmann, Ascoli, Von Dungern). They have been recently studied by Gay and Rusk. It has been difficult to explain why such sera do not spontaneously precipitate since both reacting factors are present.

In the cases of two sera recently studied by us the phenomena observed were as follows: Sera “3” and “4,” obtained by injecting two rabbits with horse serum on three successive days and bleeding eight days after the last injection, were perfectly clear and showed no spontaneous precipitation on standing several days. Serum “3” precipitated horse serum in dilutions of 1 to 1,000, and serum “4” in dilutions of 1 to 500. When mixed with equal quantities of an antihorse serum precipitin, which contained no antigen, both sera were precipitated, “3” more strongly than “4.” Neither of these sera fixed complement. When “3” and “4” were mixed, slow and slight, but distinct precipitates occurred, unmistakable after 12 hours in the ice chest.

On standing for one month in the ice chest sera “3” and “4” were found to have spontaneously precipitated. The precipitate showed slight complement fixation and the supernatant fluid was found on titration to have lost about one half of its precipitating power.

Such spontaneous precipitation of precipitin sera has been repeatedly observed after prolonged conservation.

Von Dungern who has done much work on this question has assumed a multiplicity of antigens and precipitins to account for this failure of union between antibody and antigen in such sera.