THE THEORY OF PHAGOCYTOSIS 495 



the serum to a higher content in anti-body. Again, in no case 

 is the capacity of producing anti-bodies unlimited. In certain 

 reactions the limit of possible increase is less than in others. 

 Thus it is not possible to raise the opsonic power of a serum 

 higher than a not very great multiple of its original opsonic 

 content. On the other hand, when we are dealing with the 

 reaction against bacterial toxins we find that the mechanism 

 producing antitoxin can react in an extraordinary degree, and 

 a serum many thousand times stronger than that produced 

 during the early days of immunisation may ultimately be 

 attained. The animal body also exhibits great power of forming 

 agglutinins, and the capacity of forming immune bodies seems 

 to occupy an intermediate position between the opsonic reaction 

 and the antitoxin reaction. But even in the antitoxin reaction 

 a time comes in a high immunisation when evidence of exhaus- 

 tion of the producing mechanism is manifest, so that the injection 

 of fresh toxin is no longer efficient, and the negative phase is 

 not followed by a positive phase. From the practical stand- 

 point it is the aim of the immuniser to select the time just 

 preceding such an event for the bleeding of an animal. If the 

 cells of the latter be given a few months rest then the capacity 

 for producing antitoxin usually reappears. But such facts 

 emphasise what we have said as to the possibility of every 

 immunisation entailing the infliction of an injury on some bodily 

 mechanism. 



2. The Theory of Phagocytosis. This theory, brought 

 forward by Metchnikoff to explain the facts of natural and 

 acquired immunity, has been of enormous influence in stimu- 

 lating research on the subject. Looking at the subject from the 

 standpoint of the comparative anatomist, he saw that it was a 

 verjf general property possessed by certain cells throughout 

 the animal kingdom, that they should take up foreign bodies 

 into their interior and in many cases digest and destroy them. 

 On extending his observations to what occurred in disease, he 

 came to the conclusion that the successful resistance of an 

 animal against bacteria depended on the activity of certain cells 

 called phagocytes. In the human subject he distinguished two 

 chief varieties, namely, (a) the microphages, which are the 

 " polymorpho-nuclear " finely granular leucocytes of the blood, 

 and (b) the macrophages, which include the larger hyaline 

 leucocytes, endothelial cells, connective tissue corpuscles, and, in 

 short, any of the larger cells which have the power of ingesting 

 bacteria. Insusceptibility to a given disease is indicated by a 

 rapid activity on the part of the phagocytes, different varieties 



