The plaque test for the demonstration of antibodies against the enterobacterial common antigen produced by spleen and lymph node cells.

The observations that enteric bacteria other than O14, although containing a common antigen (CA), failed to give rise to the corresponding antibodies upon intravenous injection in rabbits and that the O antigen (endotoxin, lipopolysaccharide) was responsible for this suppression prompted the study of CA antibody forming cells in spleen and lymph nodes by means of the haemolytic plaque technique of Jerne. It was shown that intravenous injection of supernates of , containing both CA and O antigen, does not result in an increase in the number of splenic cells producing antibodies against CA. In contrast, injection of partially purified CA (ethanol soluble fraction), which contains only traces of the O antigen, leads to a marked increase in the number of CA antibody forming cells. The largest number of CA antibody forming cells was present in rabbits injected first with a mixture of CA and O antigen for priming and then with small amounts of CA for booster purposes. Administration into one footpad of CA resulted in a substantial increase in antibody forming cells in the corresponding lymph node and only a small increase in those of the contralateral lymph node.