Detection of Minute Amounts of Serum Antibody by Agglutination of Antigen-Coated Bacterial Cells

Abstract

The authors recently described a local serum reaction 1 occurring upon the ear of a rabbit (at the site of a prior injection of serum antigen) when a very small quantity of homologous antiserum was injected intravenously elsewhere in the body. The amount of antiserum injected was so small that it was not possible to detect antibody in the animal's serum by the use of the conventional method of testing, i. e., the precipitation test employing the antigen dilution method. In order to determine such minute amounts of circulating antibody in the blood of injected animals it became necessary to devise a technic which would be both accurate and extremely sensitive. This was accomplished by developing a method which involves coating the cells of a suspension of Serratia marcesens with serum antigen and observing the agglutination of such cells upon addition of the animal serum being tested. That bacterial cells can be coated with serum antigen and subsequently agglutinated with an immune serum (homologous to the serum antigen) was reported by Jones. 2 However, to our knowledge, no practical application of this type of antigen-antibody reaction has been made heretofore.

It became evident in preliminary experiments, that agar grown cells (washed twice with saline solution, suspended in distilled water and killed by flowing steam) could be satisfactorily coated by suspending them in horse or beef serum (antigen) and incubating the mixture at 37°C for not less than 9 hours. With such sera in order to achieve satisfactory coating it was necessary that the quantitative ratio of serum to cells be of the order of 5 to 1, as based upon the nitrogen contents of these materials. Electrolyte content of serum does not interfere with satisfactory adsorption of antigen upon the cells.