218 PASSIVE IMMUNIZATION 



single one ever yielded 2000 units. In those animals, moreover, 

 which do yield exceptionally high values, the high tide of antitoxin 

 production is only of brief duration. As the maximum production of 

 antitoxin even under the most favorable conditions does not con- 

 tinue beyond a few months, and is then followed by a decline, in 

 spite of further immunization, it is advisable to give the animals 

 a period of three months' rest in every twelve that they are in service. 

 If this is done, the best horses furnish high-grade serum during their 

 periods of treatment for from two to four years (Park). As 6000 c.c. 

 of blood may be taken from an animal at intervals of one month, 

 it will be seen that the yield per year amounts to from 36 to 54 

 liters, allowing for the three months' trial immunization during the 

 first year and three months of rest. 



When it is desired to draw off some of the blood a superficial 

 vein of the neck is punctured with a fair-sized, sharp-pointed canula 

 and the blood allowed to flow^ through an attached tube into large 

 Erlenmeyer flasks, special pains being taken to work aseptically 

 throughout the whole procedure. The flasks are placed in a slanting 

 position before the blood clots, and kept in a cool room for three or 

 four days, when the serum which has separated out is pipetted 

 off and stored, preliminary to a bacteriological examination and the 

 determination of its titer, after which it is filled into little ampoules, 

 or into individual syringes, as the case may be, and is then ready for 

 use. In Germany, carbolic acid is used as a preservative, to the 

 extent of 0.5 per cent., while in the United States, 0.4 per cent, tri- 

 cresol is preferred, unless indeed the serum is used as such, which is 

 now frequently the case. The individual package is appropriately 

 labeled, and the date indicated, after which it should no longer be 

 used. This is necessary, as the antitoxic titer diminishes in the 

 course of time. Park states that the serum which is prepared by 

 the New York Board of Health remains within 10 per cent, of its 

 original strength for at least two months, when kept from the access 

 of air and light in a cool place, but that within a year the loss in 

 strength may amount to 40 per cent. 



Determination of the Titer. In the study of the titer of diphtheria 

 antitoxin the following standards are employed : As unit of diph- 

 theria toxin we designate that quantity expressed in fractions of a 

 c.c., which is just sufficient to kill a guinea-pig weighing 250 grams 



