Summary and Discussion
The intravenous injection of a suitable dose of killed typhoid bacilli in rabbits causes within twenty-four hours a refractory state, which enables them to withstand a dose of living typhoid bacilli that is fatal to normal rabbits. Since antibodies are not demonstrable until four or five days after an injection of antigen, this is not a process of immunization in the accepted sense of the term.
This sudden increase in resistance to living typhoid bacilli bears a certain resemblance to the cure of typhoid fever patients by crisis following the intravenous injection of typhoid vaccine, the reaction in both instances occurring in about twenty-four hours after the injection of the vaccine, which is followed in both cases by a rapid and often extremely high rise in temperature. This suggests that the same factor or factors may be concerned in the two phenomena.
In the preceding article it was shown that the injection of the doses of vaccine used in these experiments does not increase the bacteriolytic power of normal rabbit serum; hence the increased resistance in the treated rabbits is not due to this factor. Nor is it due to an increase in the number of leucocytes in the peripheral blood resulting from the preliminary injection of vaccine; for rabbits that do not show the leucocytosis are also refractory.
The bacteria disappear from the peripheral blood only slightly more rapidly in the treated than in the control animals, there being a rapid decrease in both normal and treated rabbits during the first hour. In the treated animals this decrease continues progressively until the bacteria are very scarce; in the normal controls it appears to be followed by an increase for a short time and then a rapid decrease until the bacteria are very scarce as in the treated rabbits. The cause of this difference in the normal and treated rabbits is not clear but the evidence at hand indicates that it is not due to an increase of the bacteriolytic power of the blood plasma of the treated rabbits.
For the present we wish merely to record the fact that a rabbit becomes more resistant to infection with typhoid bacilli within 24 hours after the intravenous injection of typhoid vaccine and to point out a possible relationship between the phenomenon and the cure of typhoid fever patients by means of the intravenous injection of typhoid vaccine, without attempting to set up any hypothesis as to the mechanism of the process.