172 DIPHTHERIA 



ning with very small doses, are given these animals at intervals of 

 a few days. It is desirable that the first doses should be so small that 

 marked local or general reactions are not induced. Horses differ 

 widely in susceptibility to the toxin. From time to time a small amount 

 of blood is drawn from the horse and its protective power against the 

 toxin is tested on guinea-pigs. The amount of the serum necessary 

 to protect a guinea-pig of 250 gm. weight against 100 minimum lethal 

 doses of the toxin is known as an "immunity unit." When the pro- 

 tective value of the serum has risen to the desired point, from 4 to 8 

 liters of blood are drawn under aseptic precautions into sterilized glass 

 cylinders. These are allowed to stand in a cool, dark place until 

 coagulation is complete, when the serum is treated with some preserva- 

 tive, such as 0.5 per cent, phenol, 0.4 per cent, trikresol or a small 

 amount of camphor, and placed in containers ready to be injected into 

 the sick person. In some countries no preservative is used and the 

 serum is heated to 56 C. In all countries antitoxin is made under 

 governmental supervision and the product is tested in some laboratory. 

 All horses do not produce the same grade of antitoxin, and sera 

 from different animals are mixed and the product tested as to its 

 protective value. Some horses continue to produce a high grade anti- 

 toxin for years and from 4 to 6 liters of serum may be obtained from 

 such animals monthly. It will be understood that in such cases the 

 injections of toxin must be regularly repeated. Immediately after an 

 injection of toxin the antitoxin content of the blood falls, then it rises 

 and generally reaches its highest point in ten or twelve days, remaining 

 at that point for a variable time. The horse has been immunized by 

 the repeated injections of the toxin. The process of immunization 

 consists in developing in the animal's body an antitoxin, the excess of 

 which is in the blood, and in the serum after coagulation. The anti- 

 toxin is capable of neutralizing the toxin both in vitro and in vivo. 

 The diphtheria bacillus is growing in the throat of the child and pour- 

 ing its toxin into the child's blood. The blood serum of the horse 

 is injected into the child and neutralizes the toxin coming from the 

 throat. The horse has been immunized, and when its blood serum is 

 introduced into the child, the child becomes for the time physiologically 

 a part of the horse and partakes of the horse's immunity just to the 

 extent of the amount and potency of the horse's serum injected. It 



