IMMUNITY 207 



not in use during the passage through the pigeon, and may become 

 atrophied, so that on the parasite being transferred back to the 

 fowl it will not be able to thrive owing to the loss of the receptors 

 necessary to assimilate the fowl's nutritive substances. Ehrlich 

 suggests that the majority of non -pathogenic micro-organisms, if 

 introduced into the animal body, perish by this mechanism. In 

 the case of mouse carcinoma inoculated into rats, the tumour-cells 

 proliferate for a few days, then atrophy and disappear. Ehrlich 

 suggests that some specific substance is necessary for the prolifera- 

 tion of mouse carcinoma-cells which is not present in the rat, and 

 as soon as the traces of this specific substance carried over by the 

 inoculation are used up, the cancer-cells cease to proliferate and 

 finally atrophy and disappear. These are examples of Ehrlich's 

 " atrepsy " and " atreptic immunity." 



Chauveau, in his retention theory, suggested that the 

 bacteria during their growth in the tissues form substances 

 which ultimately inhibit their growth, and, if the animal 

 recovers, prevent a subsequent development of the organ- 

 ism. The same objections may be urged against this 

 hypothesis as against Pasteur's exhaustion hypothesis. 



Bacteriolysis and phagocytosis are probably the two 

 main factors which bring about the refractory condition 

 in acquired immunity against bacteria, as well as recovery 

 from an infection. After immunisation it may be shown 

 that phagocytosis is increased, and that positive chemotaxis 

 takes place 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 leuco- 

 cytes, 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 endo- 

 thelial and fixed tissue- cells. With the colon group of 

 organisms certain humoral responses, notably agglutination 



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



