44 IMMUNE SERA 



There is absolutely no way of judging which horses 

 will produce the highest grade of antitoxin. Very 

 roughly those horses which are extremely sensitive and 

 those which react hardly at all produce the poorer grades, 

 but even here there are exceptions. 



The only way, therefore, is at the end of six weeks or 

 two months to bleed the horses and test their serum. If 

 only high grade serum is wanted, all the horses that give 

 less than 150 units per cc. are discarded. If moderate 

 grades only are. desired, all that yield 100 units may be 

 retained. The retained horses receive steadily increasing 

 doses, the rapidity of the increase and the interval of time 

 between the doses (three days to one week) depending 

 somewhat on the reaction following the injection, an 

 elevation of temperature of more than 3 F. being unde- 

 sirable. At the end of three months the antitoxic serum 

 of all the horses should contain over 300 units and in 

 about 10 per cent as much as 800 units per cc. Very 

 few horses ever give over 1000 units, and none so far has 

 given as much as 2000 units per cc. The very best horses, 

 if pushed to their limit continue to furnish blood of grad- 

 ually decreasing strength. If every nine months an 

 interval of three months' freedom from inoculations is 

 given, the best horses furnish high-grade serum during 

 their periods of treatment for from two to four years. 



Collecting the Serum. In order to obtain the serum 

 the blood is withdrawn from the jugular vein by means 

 of a sharp-pointed canula which is plunged through the 

 vein wall, a slit having been made in the skin. The blood 

 is carried by a sterile rubber tube attached to the canula, 

 into large Erlenmeyer flasks and allowed to clot, the flasks, 

 however, being placed in a slanting position before clotting 

 has commenced. The serum is drawn off after four days 

 by means of sterile glass and rubber tubing, and is stored 



