CHAPTER XV. 



NATURE OF THE PROTECTIVE DEFENCES OF THE BODY AND 



THEIR MANNER OF ACTION EHRLICH'S 



"SIDE CHAIN" THEORY. 



THE fluids and tissues of the animal body under the normal condi- 

 tions of life are, as we have seen, not only unsuitable for the growth of 

 the great majority of the varieties of bacteria, but even bactericidal to 

 the living organisms and antitoxic to their poisons. 



In seeking to account for the bactericidal property of the blood, 

 which to a greater or less extent affects all bacteria, we cannot find it 

 either in the insufficient or excessive concentration of the nutritive sub- 

 stances, nor in the temperature, nor in the reaction; for although some 

 of these conditions may be unsuitable for some bacteria, they are all 

 suitable. for many, and thus cannot constitute the fundamental explana- 

 tion of either natural or acquired immunity. We are thus driven to 

 the conclusion that the body fluids and cells contain substances which 

 are deleterious to most or all of .the bacteria. As to the origin of these 

 substances, we may conceive that they may be either regularly produced 

 by certain types of the many varieties of body cells, or that they may only 

 be produced when bacteria or other foreign cells or their substance 

 invades the body. When formed we can conceive that they may remain 

 unaltered in the fluids for a long period of time or be quickly eliminated 

 or destroyed. 



Bactericidal Properties of the Blood. The bactericidal effect upon most 

 bacteria of the blood serum, noted by Nuttall in 1888, is now undis- 

 puted, and is readily shown by the fact that moderate numbers of 

 bacteria when inoculated into freshly drawn blood usually soon die, and 

 this destruction may be so rapid that in a few hours none of millions 

 remain alive. Even when some of the bacteria survive there is for a 

 time a decrease in the number living. That this effect of the blood is due 

 to substances in the serum, and not due to serum as such, can be proven 

 by the fact that not only by injecting bacteria into the blood and peri- 

 toneal cavity, but also when the bacteria are placed in the animal body 

 after being enclosed in capsules or into serum contained in test-tubes, 

 the bacteria are killed even if they have previously grown outside the 

 body in inactive blood serum. Bacteria have also been injected into 

 a vein carefully ligated above and below, and here, without coagula- 

 tion, the blood exerts bactericidal properties. The germicidal effect of 

 any sample of blood serum on different varieties of bacteria is unequal 

 and can be watched outside of the body. Here mixed with it some 

 species of bacteria die quickly, and some lose only a portion of their 

 number, those remaining alive after a time rapidly increasing. The 



