62 CHOICE OF TOXINS 



immunized to the action of the toxin this is not the case; the 

 tissues at the site of inoculation react to the poison with the pro- 

 duction of a mass of inflammatory redema similar to that seen in 

 a horse injected with diphtheria toxin. It is obvious that these 

 connective tissues have become more susceptible to the action of 

 the tetanus toxin, and this in spite of the antitoxin with which 

 they are bathed. 



In order to avoid the difficulties arising from the occurrence of 

 hypersensitiveness in the early stages of immunization, the use 

 of unaltered toxin has now been practically abandoned, the follow- 

 ing methods, either alone or in combination, being employed 

 instead : 



1. The use of mixtures of toxin and antitoxin, the latter being 

 present in amount sufficient to neutralize all the toxin, or in 

 excess. This is repeated several times, the amount of antitoxin 

 given being gradually reduced, until at last a small amount of 

 unaltered toxin is given. 



It must not be thought that the immunity which is acquired in 

 this case is simply passive, and due to the free antitoxin which is 

 injected. The process is probably fundamentally different. We 

 shall revert to it subsequently. 



2. The injection of toxoids. This method is of especial advan- 

 tage in the case of tetanus, to which toxin animals are extremely 

 sensitive, and the dangers of the early stages of the process of 

 immunization are very great. The toxin formed in the cultures 

 may be transformed into toxoids by the action of trichloride of 

 iodine, a solution of iodine in iodide of potassium, or by heat, 

 the filtered cultures being exposed to a temperature of 60 C. for 

 a time sufficient to destroy their toxicity. Toxin that has been 

 heated to a temperature much higher than this is completely 

 destroyed, and is useless for the process. 



3. The use of serum toxin, which probably contains toxoids in 

 an unusual condition of activity. This method was introduced 

 by Cartwright Wood, and is now in general use in this country 

 for a part at least of the process of immunization, since it leads to 

 a more rapid production of antitoxin of high potency than can be 

 obtained by other methods in the same time. Ordinary alkaline 

 broth is inoculated with diphtheria bacilli, and incubated for a week 

 at 37 C. Then 15 to 30 per cent, of its volume of serum from an 

 animal of the same species as is to be used in the process of 

 immunization is added, and the incubation continued for a month 



