IMMUNITY. 181 



munized animals. Of course, when the blood serum completely dis- 

 solves its homologous bacterium, it so completely destroys it that it 

 has no longer either infectious or toxic properties. It has been 

 shown quite positively that agglutinins and lysins are not identical, 

 but the probabilities are that they represent the products of different 

 stages reached in the process of immunization. A given blood 

 serum may have an agglutinating action, and be without bacteriolytic 

 effect, or vice versa, but it must be admitted that the possession of 

 either one of these properties by the body juices militates against the 

 growth of the infectious agent, and consequently each must be con- 

 sidered as a factor in the production of artificial immunity. 



The question concerning the origin of the antiinfectious substances 

 in the production of artificial immunity is one concerning which at 

 present we can do but little more than theorize. The weight of evi- 

 dence seems to be in favor of the view that by successive injections 

 of the microorganism or its products the leucocytes are stimulated 

 to increased secretion of germicidal substances. Metschnikoff claims 

 that it is a general rule that phagocytosis is more pronounced in im- 

 munized animals than in those not immunized. He states that 

 when a microorganism is injected into an animal which has been 

 immunized to this germ, the phagocytes of the animal take up the 

 invader more promptly than is done when susceptible animals are 

 inoculated with the same germ. That phagocytosis is more marked 

 in immunized animals is shown by introducing the microorganism in 

 localities ordinarily free from phagocytes, such as the subcutaneous 

 tissue and the anterior chamber of the eye. When this experiment 

 is made, the phagocytes collect at the point of inoculation very much 

 as they do in an animal possessed of natural immunity, and there 

 they devour the invading organism. The probabilities certainly are 

 that the bactericidal substances found in the serum of immunized 

 animals originate in the phagocytes, whose capability of secreting 

 this substance is heightened by the process of immunization. Un- 

 doubtedly it is true that phagolysis also plays a part in increasing 

 the antiinfectious properties of the body juices of immunized animals. 



We now turn to a discussion of the subject of immunity to bac- 

 terial toxins. Owing to the fact that in the chapter on lysins we 

 have developed quite fully Ehrlich's views on this subject, a brief 

 statement will be all that will be necessary at this place. According 

 to Ehrlich, we are to consider that certain cells of the animal body 

 consist of a nucleus, which he designates as a " special executive cen- 

 ter," from which there radiate many side chains. It must be under- 

 stood that this is a chemical and not a morphological conception of 

 the cell. These side chains physiologically are concerned in processes 

 of nutrition. Nutritive substances brought to the cells by blood or 

 lymph can be utilized in its nutrition only after they have entered 

 into combination with one or more of the side chains. It is con- 



