PHAGOCYTOSIS 243 



by the action of the bacterial toxins, so that no phagocytosis 

 occurs ; or perhaps they may take up the bacteria and then be 

 killed by the toxins. In any case, the result is the same : the 

 bacteria continue to grow and to produce their toxins, and the 

 result is death. The animal is susceptible to the bacterium 

 because its leucocytes are unable to deal with it. Immunity, 

 therefore, is a function of the phagocytes. 



So far we have dealt with natural immunity. The application 

 of Metchnikoff's theory to acquired immunity is equally simple, 

 though much less satisfactory. He argues that the leucocytes, 

 in their contest with a particular species of bacterium, become 

 educated to overcome this bacterium, and are able to deal with it 

 in future with great ease. In the first infection there may be a 

 balanced contest of some severity and duration, but as a result 

 the leucocytes, like war-trained veterans, are readily able to cope 

 with the invader a second time. This theory, though ingenious, 

 cannot be maintained at the present day. Its truth rests, of 

 course, on the truth of Metchnikoff's main thesis, which is only 

 partially true, and which is only one factor in the complicated 

 phenomena of immunity. We may just point out, however, that 

 the life of a leucocyte is, in all probability, a comparatively short 

 one, to be measured by days, or at most weeks, so that acquired 

 immunity due to the education of the leucocytes would be of 

 short duration. Nor is it of any assistance to argue that in the 

 struggle against the invading bacterium the fittest leucocytes 

 would survive, and so lead to the general improvement of the 

 leucocyte species : for leucocytes do not propagate themselves, 

 but are emitted from the bone-marrow, run their course in the 

 blood, degenerate, and die, their place being taken by others from 

 the same source. The education, therefore, must be one of the 

 bone-marrow, and we cannot conceive how this could take place 

 as the result of phagocytosis going on in a distant area. It is 

 possible that something of the sort may occur, but only by the 

 action of toxins and other bacterial products circulating in the 

 blood-stream, and being thus brought into the marrow. Other 

 considerations might be urged, but the theory has now but a 

 historic interest. It has served its purpose : it has been the 

 means of suggesting many researches which have helped greatly 

 in the elucidation of a most difficult subject. 



Before discussing the role of phagocytosis in the infective pro- 

 cesses we must deal briefly with two subjects : firstly, the means 



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