72 IMMUNITY 



glanders. Being very susceptible to infection with tetanus while 

 undergoing treatment, a prophylactic injection of tetanus anti- 

 toxin is given each animal. McFarland found that the death 

 rate from tetanus, in a large stable, was greatly reduced after 

 using tetanus anti-toxin as a prophylactic measure. 



Immunization is started by injecting into a previously examined 

 healthy horse a mixture of toxin and an ti- toxin in which the 

 former is not quite neutralized by the latter. These quantities are 

 determined by guinea-pig .tests. Such a mixture is safer for the 

 horse and begins the immunity reaction more promptly. A 

 few doses like this are given after which pure toxin is used. 

 This is followed by a rise of temperature, local reaction and 

 systemic disturbance. After waiting for all reactions to disappear 

 injections are continued by slow increases, until, after a few 

 weeks or months, 1,000 c.c. of toxin are injected at one time 

 (enough to have killed a dozen horses that had not received the 

 smaller doses previously). The injection of the toxin is followed 

 by an immediate fall in the anti-toxic power of the serum, only 

 to be followed by a quick rise. The horse will not produce anti- 

 toxin indefinitely. After the animal has been immunized suffi- 

 ciently, his blood is drawn from the jugular vein, and after the 

 clot has formed the serum is drawn off and stored. 



Anti-toxins are found to lie in the pseudoglobulin fraction of 

 the serum. Even though an anti-serum be strong, a large quan- 

 tity would have to be injected to obtain high unit value. The 

 globulins of the whole blood are now precipitated by 30 percent 

 ammonium sulphate and the pseudoglobulins are dissolved by 

 normal salt solution. This concentrates high unit values into 

 small bulk. When removed from the horse, serum may contain 

 300-800 units per cubic centimeter. After concentration a value 

 of 3-10,000 units may be obtained. 



McFarland found that a horse was capable of producing enough 

 an ti- toxin to protect 806 other horses against doses of toxin, each 

 one of which was equivalent to the total amount of toxin that the 



