726 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
but that the blood drawn soon after the injection was made need not, 
when carefully collected, undergo putrefaction. This fundamental 
experiment, performed before pure cultures of bacteria were avail- 
able, left no doubts that the body possesses internal means of ridding 
itself of large numbers of bacteria. 
It is apparent that the body possesses two possible distinct ways of 
freeing .itself of these bacteria: It might remove them through the 
excretory organs—the kidneys or liver; it might rid itself of them by 
destroying them inside the body. It was with the rise of modern 
bacteriology that proof was brought that the blood and certain other 
body fluids—peritoneal, pleural, pericardial transudates—possess a 
remarkable power of destroying bacteria. This power resides in shed 
blood, in the other fluids withdrawn from the body, and even in the 
fluids deprived of all their natural cellular constituents. Here was 
then a concrete fact—the fluids of the interior of the body are capable 
of killing large numbers of bacteria. It could now be shown that the 
bacteria introduced in large numbers into the blood of a living animal 
“are not excreted, but are destroyed within the body. This power of 
the blood is, however, not indefinite and is not exercised equally 
against all kinds of bacteria. Even with bacteria that readily sue- 
cumb a very large number may exceed the blood’s capacity to destroy, 
so that survival and multiplication would result ; and certain bacterial 
species proved highly resistant to this blood destruction. Moreover, 
it was observed that the blood of all animals tested did not produce 
the same effects on given kinds of bacteria, that this power to destroy 
bacteria was lost spontaneously in a few days by the fluids removed 
from the body and was destroyed immediately by a temperature of 
60° C. It is, therefore, a highly labile quality. 
Apparently the way was opened up for the detection of the con- 
ditions which underlie infection and immunity and the various 
peculiarities determined by species, race, and individual. Unfor- 
tunately, there proved to be no sharp relation between the bactericidal 
powers of shed blood and immunity from or susceptibility to infec- 
tion. And important as these blood phenomena proved to be in 
accomplishing protection from infection, they do not in themselves 
account for all observed conditions. 
The factors upon which the bactericidal properties of the blood 
depend have now been clearly ascertained. The chief substance has 
been called alexin or defensive substance, but in reality the alexin is 
a compound and consists of a sensitive body—complement—and a 
more stable substance—intermediary body. Bacteria are killed and 
disintegrated when the intermediate body can attach itself to them 
and bring them under the influence of the cOmplement—a digestive 
enzymotic element, to which the intermediary body also attaches 
itself, 
