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 carefully 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 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 This power resides in shed blood, in the other fluids withdrawn from the body, and even in the fluids deprived of all their natural cellular constituents.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 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 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 conditions which underlie infection and immunity and the various peculiarities 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 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.