[568] CHAPTER XYII 



SUMMARY 



Means of defence of the animal against infective agents. — Absorption of micro- 

 organisms, — Phagocytes, and their function in inflammation. — The action of 

 phagocytes in the absorption of micro-organisms. — The cytases, phagocytic 

 ferments. — The cytases are closely bound up with the phagocytes. — The fixatives 

 and their function in acquired immunity. — The fixatives are excreted by the 

 phagocytes and pass readily into the fluids of the body. — Essential mechanism 

 of the action of the fixatives. — Adaptation of phagocytes to destroy micro- 

 organisms in acquired immunity. — DiflFerence between the fixatives and the 

 agglutinins.— Antitoxins and their analogy with the fixatives. — Hypotheses as 

 to the origin of antitoxins. — Cellular immunity is a fact of general import. — 

 Susceptibihty and its role in immunity. — Applications of the theory of immunity 

 to medical practice. 



When an animal remains unharmed in spite of the penetration of 

 infective agents it is said to be immune to the diseases usually set 

 up by these agents. This idea embraces a very great number of 

 phenomena which cannot always be sharply separated from allied 

 phenomena. On the one hand, immunity is closely connected with 

 the process of cure, on the other, it is related to the disease. An 

 animal may be regarded as unharmed if the penetration of a very 

 dangerous virus sets up merely an insignificant discomfort. Never- 

 theless, this discomfort is accompanied by morbid symptoms, though 

 they may be very slight. It is useless and impossible to set up any 

 precise limits between immunity and allied states. 

 . Immunity presents great variability. Sometimes it is very stable 

 ^and durable ; in other cases it is very feeble and transient. Immunity 

 may be individual or it may be generic. It may be the privilege of 

 a race, of a species. 



Immunity is often innate, as is the case of the immunity which is 



^ called natural. But it may also be acquired. This last category of 



immunity may be developed either by natural means, after an attack 



of an infective disease, or as a result of human intervention. The 



