IMMUNITY TO TOXINS 127 



that in immunizing a susceptible animal (rabbit or guinea-pig) 

 to tetanus the connective tissues acquire receptors suitable for 

 the toxin in question. They will absorb this toxin, be shed, and 

 the animal will develop antitoxin, whilst the tissues previously 

 susceptible the brain and cord will remain as susceptible as 

 before. This is probably what occurs, though the critical proof 

 the demonstration of an increase in the toxin-absorbing power of 

 the tissues in acquired immunity, and thus of an increased number 

 of receptors does not appear to be forthcoming. Dmitrevsky 

 investigated the amount of tetanus toxin neutralized by brain 

 tissue before and after immunization ; but since, as we have seen, 

 this process is probably different in nature from the neutralization 

 of toxin by antitoxin, his researches have not much weight in this 

 connection. 



The third explanation is that of Metchnikoff, that the leucocytes 

 themselves have become immune to the toxin, and have thus 

 acquired the power of dealing with that substance in large 

 quantities, so that the tissues are sheltered from its action. This 

 theory simply shifts the immunity back to the leucocytes, and 

 thus does not really solve the problem, which, it must be 

 admitted, is unsolved whatever theory we may adopt, and perhaps 

 insoluble in the present state of our knowledge of the physical 

 chemistry of living matter. We may, however, combine it with 

 perfect propriety with the side-chain theory, and argue that the 

 leucocytes may have behaved in the same way as we have 

 supposed the tissue cells to have done in the last paragraph that 

 is, to have retained, and perhaps increased, their receptors, whilst 

 losing their susceptibility to the toxophore group. Here, again, 

 we are confronted with the absence of any definite evidence either 

 way. There seems to be no experimental proof to show whether 

 leucocytes from a hypervaccinated animal have any increased 

 resistance to the poisonous action of toxins or any increased 

 power of combining with them. This is greatly to be deplored, 

 and might easily be remedied now that Sir Almroth Wright has 

 taught us a simple and convenient method of working with living 

 leucocytes. As far as our experimental knowledge goes, there is no 

 great difference in leucocytes from immunized and normal animals, 

 and the researches on opsonins have led to a tendency to disregard 

 the possible variations in the immunity of the leucocytes, since 

 the researches of Bulloch and others showed that leucocytes from 

 very varied sources would take up the same number of bacteria 



