84 THE BACTERICIDAL SUBSTANCES OF THE BLOOD 



and then introducing the minimal dose of bacteria which in the 

 untreated animal would invariably produce a fatal result. It will 

 now be seen that the animal does not succumb, and if a series of 

 corresponding experiments be carried out it can be demonstrated 

 that a number of multiples of the originally just infecting dose must 

 be injected in order to kill. Weil has shown very satisfactorily that 

 this result is purely referable to the action of the leukocytes and not 

 to any bacteriolysins that may be present, by previously rendering 

 the latter inactive with so-called complement-binding substances, 

 when infection in the untreated animal may be brought about with 

 subminimal infecting doses (as compared with a control animal), 

 while in one that has been previously rendered hyperleukocytic this 

 effect is not obtained. 



Upon the basis of analytical studies such as those outlined in the 

 foregoing pages, incomplete as they are, we can now distinguish three 

 different types of infection (excluding those with the so-called necro- 

 parasites, in which a successful infection can scarcely be brought 

 about under ordinary conditions). In the first, represented by the 

 anthrax bacillus, the serum in itself is either inactive or shows but 

 slight inhibitory qualities, while the combination of serum with leuko- 

 cytes has strong antibacterial properties which can be completely 

 overcome, however, through the aggressivity of the organism. 



The second type is represented by various streptococci, staphylo- 

 cocci and certain vibrios (el Tor.), i. e., organisms which stand very 

 close to the group of the true parasites. In such infections the serum 

 alone manifests but little bactericidal effect, while the antibacterial 

 action of the serum, when combined with leukocytes, is strongly 

 marked and can be only partially overcome by the aggressivity of the 

 organism. In the third type, the serum alone, as well as in combina- 

 tion with leukocytes shows marked antibacterial properties, the 

 former by itself being sometimes sufficient to offset the aggressivity 

 of the corresponding bacteria. The animalization of the organisms 

 is here of little avail, as a protective measure against phagocytosis, 

 while it is partially effective in the case of the serum. Most members 

 of the typhoid and the vibrio group fall under this category. 



