8 THE POPULAR SCIENCE MONTHLY 



putrescent fluids into the veins of rabbits and noting that not only 

 might they survive the infections and remain quite normal, but that the 

 blood drawn soon after the injection was made need not, when care- 

 fully collected, undergo putrefaction. This fundamental experiment, 

 performed before pure cultures of bacteria were available, left no doubts 

 that the body possesses internal means of ridding itself of large num- 

 bers 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 re- 

 markable 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 blood, in the other fluids withdrawn from the body, and even in the 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 succumb 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 spontane- 

 ously 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 condi- 

 tions which underlie infection and immunity and the various peculiari- 

 ties determined by species, race and individual. Unfortunately, there 

 proved to be no sharp relation between the bactericidal powers of shed 

 blood and immunity from or susceptibility to infection. And impor- 

 tant 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 disin- 

 tegrated when the intermediate body can attach itself to them and 

 bring them under the influence of the complement — a digestive en- 

 zymotic element, to which the intermediary body also attaches itself. 



