68 CLINICAL BACTERIOLOGY 



why some animals are more predisposed to specific infec- 

 tions than others. 



But, of course, it must be pointed out that a high opsonic 

 power does not necessarily mean absolute immunity against 

 bacterial invasion, for, as we have already seen, there are 

 other protective elements in the blood, and in addition the 

 living tissue and organic structures of the body, which 

 play their part in the role of conferring and sustaining 

 immunity, and to what extent each can act by itself is at 

 present not known. 



Phagocytosis. 



The first important theory of disease resistance was put 

 forward by Metchnikofr, who studied the behaviour of the 

 leucocytes in the lower animals, and attributed the destruc- 

 tion of bacteria to their activities. MetchnikofPs theory 

 is that the phagocytes approach and devour invading 

 bacteria, and if they succeed in doing this the disease is 

 checked and prevented. It has been already pointed out 

 that the leucocytes are not in themselves capable of acting 

 in the capacity of invaders, but, as Wright and others 

 believe, the bacteria must be in some manner prepared for 

 them before digestion can take place. On the face of this 

 it appears only reasonable to suppose that phagocytosis is 

 only one link of the complex chain which every living 

 being is daily forging within his economy in his fight 

 against more or less constant bacterial invasions ; and it is 

 to the harmonious working of all these protective forces, 

 presenting a strong, vigorous front, that we owe and main- 

 tain our existence. 



