IMMUNITY AND ITS VARIETIES. 97 



by the action of the toxins of the bacteria on the animal ; 

 and to be antimicrobic, when it has been obtained by means 

 of the action of virulent or attenuated cultures on those 

 animals. 



Uses. Antitoxic serum is employed chiefly in the toxic 

 diseases, such as diphtheria, tetanus, etc., and antimicrobic 

 serum is used particularly in the invasive diseases, such as 

 plague, typhoid fever, cholera, etc. 



Theories in Explanation. A number have been suggested. 

 Some believe that the antitoxin is a chemically changed toxin ; 

 others claim that it is a sort of enzyme produced by the toxin ; 

 others again state that it is the product of the cytic activity 

 developed by the toxin; again others consider that it acts as 

 a sort of combining ferment in the same manner as those fer- 

 ments which favor coagulation of the fibrin in the blood. 



The Theories of Immunity. 



How these substances act so as to produce immunity in ani- 

 mals is a subject that has occupied investigators considerably in 

 recent years. 



I. The abstraction theory (Pasteur's) is to-day only of his- 

 torical interest. It was believed to be due to the fact that 

 the pabulum necessary for the life of the specific bacteria had 

 been consumed, and that these bacteria could no longer live 

 in the animal. 



II. The retention theory (Chauveau's), in which it was sup- 

 posed that microorganisms left in the system certain substances 

 which were antagonistic to their further growth, is still worthy 

 to-day of some consideration. 



III. The theory of phagocytosis (Metchnikoff's), by which 

 immunity was supposed to be due to the action of the white 

 blood-corpuscles, which have the power of absorbing and 

 destroying bacteria, is not tenable to-day in its original 

 entirety. That the leucocytes play a certain part in the 

 immunizing process cannot be denied, but the phagocytic 

 property is more probably due to the fact that the animal is 

 immune than the cause of the immunization. 



7 M. B. 



