On the Possible Importance of Colloidal Protection in Certain Phases of the Precipitin Reaction

It 'has been frequently observed by those who have worked with l:he precipitin reaction that the sera of protein-immunized animals may contain, at certain times, both precipitin, and, side by side with it, unassimilated remnants of the antigen. Such sera, w'hen taken, are clear, but will show precipitation not only when mixed with dihttions of the antigen, but also when added to other homologous precipitating sera. This phenomenon has been noticed by Linossier and Lemo~ne ( I ) , Eisenberg (2), Ascoli (3), and others, and has been extensively studied by yon Dungern (4). Gay and Rusk (5) have recently observed it in connection with the rapid method of precipitin production of Fornet and Mfiller (6), and have noted that such sera, although containing both antigen and precipitin, do not possess complement-fixing properties. According to Uhlenhuth and Weidanz (7), the antigen may persist in the sera of proteinimmunized animals, in demonstrable amounts, as long as fifteen days after the last injection, and it is constantly present during this period, but in progressively diminishing amounts. We are thus confronted by the apparently paradoxical phenomenon of the presence in these sera, side by side, of an antigen and its homologous precipitin, incapable of rea.cting with each other, although each of them readily reacts with precipi.tin or antigen respectively, when these are added from another source. Many attempts have been made to a,ccount for this. A number of observers, notably Eisenberg, have concluded, from extensive analyses of quantitative relationships, both of agglutinin and precipitin reactions, that these take place according to the laws of mass action. In consequence, in addition to the combined precipitin-anti-