198 STUDIES IN IMMUNITY. 



the corpuscles properly. It will be recalled that in these experi- 

 ments we employed an active hemotoxin from guinea-pigs that have 

 received three injections of 4 to 5 c.c. of rabbit blood. 



We may note at once that the antitoxic serum is in reality more 

 active than would seem from this preliminary experiment; by a 

 very simple procedure it may be made to neutralize much larger 

 quantities of hemolytic serum perfectly. If the serum is heated 

 to 55 degrees it is found that three or even two volumes of anti- 

 toxic serum will completely neutralize one volume of fresh hemo- 

 lytic serum. 



As we shall presently see, the explanation of this fact is simple. 

 When the antitoxic serum is fresh it contains an excess of rabbit 

 alexin, which, in the presence of the sensitizing substance, is dan- 

 gerous for the rabbit corpuscles. In order, therefore, to demonstrate 

 its maximum protective power one should eliminate this harmful 

 substance by heating the serum to 55 degrees. 



That the antitoxic serum does possess an alexin capable of de- 

 stroying rabbit corpuscles when the sensitizing substance is present 

 as well as normal rabbit serum, is easily demonstrable. On mixing 

 hemolytic serum heated to 55 degrees with equal parts of fresh anti- 

 toxic serum and adding a small amount of rabbit blood we find that 

 the corpuscles are dissolved. This destruction is due to the alexin 

 in the antitoxin since there is none in the hemolytic serum. If we 

 should add too great a dose of antitoxin to the sensitizing substance, 

 there would be no hernolysis. For, as we shall see, the sensitizing 

 substance will have been neutralized and under these conditions 

 the alexin in the antitoxin can have no harmful effect on the 

 corpuscles. In the presence of the sensitizing substance, then, a 

 small dose of antitoxin destroys the corpuscles but a large dose 

 protects them. 



In order, then, to study the properties of the antitoxin to best 

 advantage it is better to heat the serum to 55 degrees as a general 

 procedure. 



Antisensitizing substance and anti-alexin. We know that hemo- 

 lytic serum destroys rabbit corpuscles owing to the action of two 

 distinct substances : the specific sensitizing substance (antibody or 

 preventive substance) and the alexin. This latter substance, which 



