IMMUNITY AND ITS VARIETIES. 101 



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. 



Immunity is, in general terms, certainly produced by certain 

 secretions formed in the animal's body, and secreted by it to pro- 

 tect itself from the attack of the invading bacteria, and distrib- 

 uted in all the tissues, but found especially in the serum of the 

 blood. 



IV. Ehrlich's Side-chain Theory. Ehrlich, studying the ef- 

 fects of antitoxin on toxin, concluded that a definite chemical 

 combination, with an inert body as the product, resulted from 

 the union of one with the other. It was his further opinion 

 that the specific nature of such reactions tetanus antitoxin 

 uniting only with tetanus toxins, diphtheria antitoxin only 

 with the 'toxin of diphtheria showed that certain special 



