132 PASSIVE IMMUNITY 



milk (in which the antibodies have not been destroyed by heat) in 

 the feeding of infants. It seems also that absorption of antitoxins 

 can take place in the intestine, and that the process may occur 

 when gastric digestion is impaired or is prevented by artificial 

 means ; thus McClintock and King obtained successful results (in 

 animals) in 93 per cent, of cases after the administration of a 

 mixture of diphtheria antitoxin, opium, and of a saturated solution 

 of salol in chloroform. It is possible that something of the same 

 sort may occur in disease where the digestive powers are notably 

 enfeebled, but this does not justify its exhibition in this way in 

 cases of disease where every hour is of the utmost value. Recent 

 researches have also shown that in some cases animals may be 

 immunized per rectum ; possibly success is only attained when the 

 antitoxin goes sufficiently high up. 



As regards the duration of the immunity conferred by a single 

 dose of antitoxin, our knowledge is not very exact in the case of 

 human beings. According to von Behring, Goodman, and others, an 

 animal injected with diphtheria antitoxin retains its immunity for 

 rather more than three weeks, and the duration is not markedly 

 affected by the size of the dose given for example, 20 units of 

 antitoxin per kilogramme immunized a rabbit twenty-three days, 

 500 units twenty-six days. According to Goodman, the degree of 

 immunity falls rapidly for the first two or three days, and then more 

 slowly. According to him also, the immunity is greater than the 

 amount of antitoxin in the blood would lead one to suppose, and it 

 continues when antitoxin is no longer demonstrable in that situation. 

 The duration of the immunity appears to depend upon the source of 

 the antitoxin injected, being longer if the serum is homologous 

 i.e., derived from an animal of the same species : thus von Behring 

 showed that horses immunized by horse-serum antitoxin retain 

 their passive immunity almost as long as if they had acquired 

 active immunity from toxin injections, whereas the effect in 

 rabbits, etc., as we have seen, is more transient. 



But little need be added to what has been already mentioned 

 incidentally with regard to the curative power of antitoxin. It 

 acts, in the main, by neutralizing all toxin present in a free state 

 in the blood or subsequently formed by the bacteria. In general, 

 as has been pointed out, it does not repair the damage that the 

 toxin has already done, nor remove the latter from its combination 

 with the susceptible cells. It is perhaps hardly safe to assume that 

 there is no trace of such an action, and it is by no means improb- 



