Discrepancies Between strong immunochemical relations of S. typhimurium and S. kentucky and their behaviour "in vivo".

Proteins from S. typhimurium and from S. kentucky strongly cross-precipitated against sera prepared in rabbits with the respective strains; agglutinations paralleled the precipitations. The related antigens were, however, not involved in "in vivo" cross-protections. Mice immunized with proteins from S. kentucky resisted the toxicity of the homologous strain and that of a concentration of S. emek which killed the controls but did not survive the infection with 1LD100 of S. typhimurium. There was a long delay in the death of those mice infected with S. typhimurium while the mice immunized with S. typhimurium proteins were killed by the toxicity of 1LD100 S. kentucky, like the controls, within 20 hours. The results strikingly underline the specificity of host-parasite relations and the need for circumspection when involving common antigens in either virulence or toxicity.