Pasteurella pestis bacteria crossreact serologically with Pasteurella pseudotuberculosis R-sera but not with OH-- or O-sera. The common R-antigen is soluble in water. Virulent plague strains seem to possess a Vi-antigen. A virulent pseudotuberculosis strains gave full protection against massive doses of Pasteurella pestis. The serological relationship between Pasteurella pseudotuberculosis and Pasteurella pestis R-antigen has its immunological counterpart. The water-soluble fraction IB of Pasteurella pestis Baker et al. contains a component common with Pasteurella pseudotuberculosis strains of V groups. This component can be eliminated from fraction IB serum either by absorption with Pasteurella pseudotuberculosis bacteria or Pasteurella pestis R-strains. It is suggested that the immunochemically common factor is responsible for the immunity induced. Author
Trans. of Insitut Pasteur, Paris. Annales (France) v91 p68-74 1956.