254 A MANUAL OF BACTERIOLOGY 



chemotaxis is evinced by the leucocytes towards the 

 organism, whereas previously negative chemotaxis 

 occurred ; the leucocytes have been " educated," as it 

 were, to be attracted, instead of repelled, by the bacterial 

 invasion. According to Andrewes, 1 the defence against 

 the pyogenic cocci is not only essentially phagocytic, and 

 dependent upon the polynuclear leucocytes, but is also, in 

 the main, opsonic. In tuberculosis and syphilis the 

 polynuclear leucocyte takes little part in bodily defence, 

 which is essentially a function of the endothelial and 

 fixed tissue-cells. With the colon group of organisms 

 certain humoral responses, notably agglutination and 

 bacteriolysis, are better marked than with most other 

 bacteria, and polynuclear phagocytosis seems subsidiary. 



Antitoxin formation probably plays little or no part 

 in acquired immunity, or even in recovery from infection. 

 In diphtheria, for instance, antitoxin is not found until 

 the disease has subsided. Possibly, in chronic infections, 

 antitoxin formation does play a subsidiary role in recovery. 



To sum up, natural immunity is probably due to a 

 number of factors, some or all of which may be operative 

 in particular instances, and it is impossible to state with 

 certainty any general law. In most cases phagocytosis 

 is the principal means of defence, the germicidal, inhibi- 

 tory, or bacteriolytic actions of the body -fluids aiding, 

 though of subsidiary importance ; in others the cells and 

 tissues are unaffected by the bacterial toxins, sometimes 

 because the cells are lacking in the particular side-chains 

 or receptors which fix the toxin, sometimes because, for 

 some unknown reason, the cells are unaffected by the 

 toxophore group of the toxin. 



As regards the immunity acquired after an attack of 

 disease, this may be due to the " education " of the leuco- 



1 " Croonian Lectures," Lancet, June 25 et seq., 1910. 



