IMMUNITY. 169 



microorganism. What this chemical poison is, or whether it is the 

 same in all phagocytes, we cannot at present determine. All phago- 

 cytes contain nucleic acid, and the germicidal properties of this have 

 been abundantly demonstrated, but whether or not there may be 

 other and more powerful bactericidal agents in certain phagocytes, we 

 have no means at present of knowing. It is still a question whether 

 or not the germicidal substance contained in the leucocytes is a secre- 

 tion of these organisms or a result of their disintegration. As has 

 already been stated in the chapter on the germicidal constituents of 

 the blood, it seems quite evident that the bactericidal substances in 

 the blood serum result from the disintegration of the white corpuscles. 

 It is possible that a blood which furnishes a highly bactericidal serum 

 gives off but little or no germicidal substance to the plasma in the 

 living body. It is probable that the leucocytes in the blood of 

 certain animals disintegrate more rapidly when removed from the 

 body than happens to the leucocytes of other animals. In instances 

 of the first kind we would expect to find the blood serum possessed 

 of marked germicidal properties, while in cases of the second kind 

 the serum may be quite devoid of bactericidal action. Again it is 

 possible that the phagocytes in the blood of one animal may retain 

 their germicidal constituents longer than do the leucocytes in the 

 blood of other animals. In the serum of a blood of the first kind we 

 would not expect to find a large amount of germicidal substance and 

 yet within the body the phagocytes of this animal may be more 

 effective in the destruction of germs than are those in the blood of 

 an animal whose serum is rich in germicidal substances. 



There is another chemical process in the phenomenon of phago- 

 cytosis which is of the greatest importance. The phagocytes are 

 attracted by the introduction into the body of certain substances and 

 repelled by others. This is known as chemotaxis, which may be 

 either positive or negative. In other words, the foreign substance 

 introduced into the animal body may attract the phagocytes, or it 

 may repel them. In the former instance the phagocytes will gather 

 in large numbers about the foreign substance introduced into the 

 body, and phagocytic action will be most marked. It appears to us 

 that the greatest factor in natural immunity, so far as the action of 

 the blood of the body on the invading microorganism is concerned, 

 lies in the fact that in natural immunity chemotaxis is positive, and 

 the more marked the natural immunity is, the stronger is the posi- 

 tive chemotaxis. It should be understood that at present we have 

 reference only to natural immunity against infection, and that we do 

 not include in these statements our opinions concerning natural im- 

 munity against toxins. 



That bacteria may retain their vitality after being taken into the 

 phagocyte has been demonstrated by injecting phagocytes containing 

 bacilli into susceptible animals and thus inducing infection. When 



