QUANTITATIVE STUDIES ON THE PRECIPITIN REACTION THE R6LE OF MULTIPLE REACTIVE GROUPS IN ANTIGEN-ANTIBODY UNION AS ILLUSTRATED BY AN INSTANCE OF CROSS-PREcIPITATION* BY MICHAEL HEIDELBERGER, Pa.D., AND FORREST E. KENDALL, Pa.D

The writers have been studying an antigen-antibody system in which the antigen was the red dye, R-salt-azo-benzidine-azo-crystalline egg albumin 1 (1, 2), fractionated to the extent that it reacted only exceptionally with anti-egg albumin sera. However, as stated previously (2), the original specificity was not entirely abolished, since antisera to the dye protein yielded precipitates with crystalline egg albumin. A quantitative comparison of the dye-antidye and egg albumin-antidye reactions disclosed great differences, as will readily be seen from Table I .and the corresponding Fig. 1. In the latter the solid curve (I) represents the amount of antibody ([total N antigen N] × 6.25) precipitated per cubic centimeter from an antidye rabbit serum by varying amounts of dye protein; the broken curve (II) shows the amount of antibody precipitated by varying amounts of egg albumin from the same serum, while the dotted curve (III) represents antibody precipitated by egg albumin from antiegg albumin of similar precipitin content. The conclusions regarding antigen-antibody interaction drawn from the data will be discussed below.