IMMUNITY AND SUSCEPTIBILITY. 551 



mains uninfected, two possibilities present themselves: first, the presence 

 of natural antitoxins and second, the presence of antibacterial sub- 

 stances. It is necessary, of course, to have excluded the possibility 

 of natural antitoxins, it having been demonstrated that the organism 

 injected produces its diseased effects by endotoxins held within the 

 bacterial cell rather than by toxins. The antibacterial action of the 

 blood may be due to two constituents, namely, cellular substances 

 (leucocytes) and serum substances. The rat and the dog are both 

 immune to anthrax, but the immunity of the dog is not due to anti- 

 bacterial substance but to the phagocytic activity of the leucocytes, while 

 in the rat the immunity is due not to the leucocytes but to the anti- 

 bacterial substance. 



Antibacterial action is due to two substances in the serum: first, the 

 thermostable substance which combines with the bacteria, called an 

 amboceptor, and second, a thermolabile substance, called a complement 

 which combines with the amboceptor after this substance has combined 

 with the bacterial cell. It is sufficient to say at this time that these 

 substances do occur in normal sera and that the result of their com- 

 bination with the, bacterial cell causes the death of the bacteria and in 

 some cases a lysis (solution) of the bacteria in addition. 



There may be present in the body of animals antibacteria sub- 

 stances of three kinds: first, those just killing bacteria (bactericidal), 

 second, those killing the bacteria and dissolving them (bacteriolytic) 

 and third, leucocytes which are active in the ingestion of the specific 

 microorganisms. In all probability the overactivity of leucocytes in 

 every case of natural phagocytic immunity is due to the presence of 

 normal opsonins, substances which sensitize the bacteria and render 

 them susceptible to phagocytosis. 



Normal Hemolysins. Normal hemolysins (hemoglobin-liberating 

 substances) are present in the sera of certain animals for the red blood 

 corpuscles of other animals of different species, and the same species, 

 but never for the red corpuscles of the animal from which the serum was 

 obtained. Such substances are known respectively as heterolysins, isoly- 

 sins, and if the last occurred the name autolysins would be applied. The 

 hemolysins will be discussed later. 



Normal Agglutinins. Normal agglutinins for various bacteria, such 

 as B. typhosus, Msp. comma, Bad. dysenteric, B. coli and Ps. pyocyanea 

 are present in the blood serum of animals. It is necessary, of course, 



