IMMUNITY IN TUBERCULOSIS—FLEXNER. 643 
I am in the fortunate position of being able to bring before you a 
critical summary of the subjects just presented by one wholly con- 
versant with its practical as well as its theoretical aspects. Through 
the courtesy of Dr. Leonard Pearson I have been enabled to read the 
advance sheets of a review on immunization in tuberculosis which will 
soon be issued from the Phipp’s Institute. Doctor Pearson concludes 
that there appears to be no doubt that different cultures of human 
bacilli have different immunizing values. Some can not be used at all 
because they are of too high, and others, possibly, because they are of 
too low, virulence for cattle. There is also need for comparison in 
immunizing value of fresh cultures and cultures that have been dried 
in vacuum and reduced to powder. Some observations appear clearly 
and strongly to indicate that the fresh cultures are preferable. Al- 
though it has been shown that vaccination can be practiced so as to be 
entirely harmless to the animals, yet, on the other hand, it is not 
always unattended with danger. What is the shortest and most 
economical procedure for the protection of cattle on a large scale is 
still to be- established. Only prolonged observation of carefully 
recorded results of vaccinations practiced on a large scale can settle 
this point. The question of duration of immunity is still an open 
one. It has been shown that the immunity endures a year. To say, 
at the present stage of the studies, that it will last during the entire 
life of an animal is to make a statement for which there is no experi- 
mental proof. Modes of vaccination, as illustrated by the intervals 
between the successive injections, differ greatly. Behring recommends 
an interval of three months, while others have obtained a high degree 
of immunity by repeated injection at short intervals. As artificial 
immunity is relative and not absolute it need not excite surprise that 
the immunity to the tubercle bacilli can be overcome by the injection 
of large quantities of active bacilli. What is desired in practice is a 
degree of immunity that will suffice to protect animals from acquiring 
the disease under natural, and consequently highly variable, condi- 
tions. In some herds, where the natural disease prevails in a mild 
form, a lower degree of immunity may suffice than in other herds, in 
which the disease is more severe and widespread. We are, therefore, 
at the beginning of this complex and highly important subject. These 
are Doctor Pearson’s conclusions. 
There is another aspect of this subject which demands attention. 
When it is recalled that immunity in cattle is obtained by the injec- 
tion of living human tubercle bacilli the question arises whether this 
procedure is wholly free from danger to the consumers later of the 
flesh and milk of these cattle. It would appear that the human bacilli 
do not excite in cattle the tubercular lesions, in which doubtless the 
bacilli are so inclosed as to be, to a considerable degree, protected 
from perishing. It is equally true that as the living micro-organism 
