A study has been made of the degree of demonstrable cross-immunity which a monovalent Typhoid, Para A, and Para B vaccine will produce in experimental animals toward the two heterologous organisms of this trio of intestinal pathogens. The polyvalent protective potency of a few sera from humans initially vaccinated with monovalent typhoid vaccine, against five common types of Salmonella, was also determined.

In one phase of this study, three respective groups of mice were actively immunized with each of the three vaccines, and each group was subsequently tested for immunity to graded doses of E. typhosa, S. paratyphi, and S. schottmuelleri.

The results of these tests for active immunity showed that mice immunized with a relatively large dose of Typhoid vaccine acquired thereby sufficient hetero-immunity to survive 10 MLD of S. schottmuelleri; and that the same dosage of Para B vaccine protected mice against the same multiple of the MLD of E. typhosa. However, neither of these two organisms, when prepared as vaccines and inoculated into mice, produced a significant degree of immunity in these animals to S. paratyphi; nor did Para A vaccine, inoculated into mice, produce in them any significant degree of immunity to E. typhosa or to S. schottmuelleri.

In the second phase of this work, respective groups of mice were passively immunized by the intraabdominal administration of Typhoid-, Para A-, and Para B-immune rabbits' sera, and each group was subsequently subjected to the administration of graded doses of E. typhosa, S. paratyphi, and S. schottmuelleri in order to measure the degree of immunity to each of these organisms that was conferred on the animals by each of the immune sera.

These measurements of passive immunity demonstrated that when rabbits were inoculated with a vaccine prepared with any one of these three organisms, the resultant serum, when administered to mice in 0.1 ml amounts, conferred on them a high degree of immunity to the heterologous organisms. This was especially true of the cross-immunity demonstrated between E. typhosa and S. schottmuelleri, and to only a slightly lesser degree between S. paratyphi and each of the other two organisms.

The immune substances produced in human sera by monovalent typhoid vaccination are apparently of the same order as those produced in rabbits by the same procedure, and possess a demonstrable protective potency against antigenically related types of Salmonella.

It is difficult to say which of the two determinations—active or passive immunization-tests—is the more reliable criterion for the appraisal of cross-immunity between the organisms used in these experiments. It is believed, however, that in an investigation of this kind, both types of tests should be made and reported in order to present the complete immunological relationship. The somewhat conflicting results do not detract from the value of the experimental work, but amply illustrate how conflicting conclusions could be drawn if two independent workers employed different methods, instead of both methods.

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