266 THE DIPHTHERIA BACILLUS 



the directions given on pp. 49 and 50 (Nocard's method and Latapie's 

 apparatus). Six litres of blood yield nearly 4 litres of serum. 



The horse is maintained in a state of immunization by the inoculation of 

 toxin from time to time. 



When several animals have been immunized it is highly desirable that the serum 

 of the various horses should be mixed ; by doing this a product is obtained of which 

 the antitoxin content is uniform. Moreover, the serum of some horses is liable to 

 provoke erythematous rashes when used in the human subject, which though harm- 

 less are nevertheless irritating, and by mixing different serums this inconvenience 

 may be minimized. 



For the purpose of storing serum it is distributed with aseptic precautions 

 in small sterile bottles stoppered with sterile india-rubber plugs and kept 

 in the dark. 



Serum prepared under strictly aseptic precautions may be kept in these 

 climates many months in a sterile condition without losing any of its anti- 

 toxic properties. Occasionally the serum after bottling becomes distinctly 

 cloudy, but this is of no importance with respect either to the purity or 

 efficacy of the serum. A deposit is less likely to occur and the keeping pro- 

 perty of the serum is better assured if, immediately after filling, the bottles 

 are heated for an hour at 57 C. in a water bath. This degree of heat has no 

 effect upon the properties of the serum. 



Dried serum. Serum may be dried by evaporation in vacuo. Just before 

 use the dried serum is dissolved in eight or ten times its volume of sterile water ; 

 this solution frequently gives rise to a local but transient swelling which is not the 

 case with liquid serum. In these latitudes liquid serum should always be admini- 

 stered in preference to the dry product : the value of the latter is apparent in warm 

 climates where liquid serum quickly loses its properties. 



Antitoxic milk. The milk of immunized females possesses antitoxic properties 

 (Ehrlich). This fact however is merely of theoretical interest because the extreme 

 dilution of the antitoxin in the milk renders the latter incapable of being used in 

 practice. 



Still, milk containing antitoxin may be condensed to a sufficiently small volume 

 to allow of laboratory experiments being conducted with it (Wassermann). The 

 milk of cows or goats can be used ; for experimental purposes it is scarcely possible 

 to get a milk with a preventive strength of one-fifth (vide infra, p. 268). 



(b) Properties of the serum. 



The serum of immunized animals is antitoxic, that is to say if the serum be 

 mixed with toxin in suitable quantities the mixture is harmless on inoculation 

 into animals. 



This property of the serum is due to a special substance known as Antitoxin, the 

 nature of which is as little understood as is the nature of toxin : like toxin, antitoxin 

 is altered by heat, precipitated by alcohol and carried down with the precipitates 

 formed in liquids which contain it in solution. In the living body it is formed in 

 response to the absorption of toxin ; " under the influence of toxin, certain cells 

 of the living body acquire a new and persistent secretory property " (Salomonsen 

 and Madsen). 



Antitoxin saturates toxin both in vivo and in vitro (p. 224) : it has both 

 prophylactic and curative properties : a guinea-pig inoculated with an adequate 

 dose of serum can withstand the subsequent inoculation of such a quantity 

 of toxin as would be sufficient to kill with certainty a non-inoculated guinea- 

 pig. Even if the toxin be inoculated first and the serum not until several 

 hours later, the animal will be protected. Immunity is rapidly produced 

 but is short-lived : in a few days or weeks it has entirely disappeared. The 

 amount of serum necessary to cure an animal inoculated with toxin depends 

 upon many factors : among others upon the weight of the animal, the amount 



