54 
We see from the above series that it required 17 days (or less) 
for the guinea pigs to become susceptible. 
We learn from these three series that guinea pigs receiving 4 c. c. 
of antitoxic horse serum are apparently not quite so susceptible as 
those receiving the smaller amounts (g-Jo c. c. and 1 c. c.); also, 
that those which received 1 c. c. are apparently not quite so sus- 
ceptible as those which received only c. c. of the horse serum. 
From our work we know that it is the quantity of horse serum, and 
not the number of units contained in the serum, which plays the 
important role. If this relation of the amount of serum is true, we 
presume that animals receiving large quantities of horse serum for 
the first injection are less susceptible because they are rendered 
slightly immune, just as animals may be immunized against this 
action by daily repeated inoculations. When large quantities are 
given at the first injection, the horse serum is probably absorbed 
very slowly and acts somewhat in the same manner. 
It would also seem from the above that guinea pigs are not ren- 
dered quite as susceptible by first injections of horse serum as by 
injections of the toxine-antitoxin mixture. 
THE SENSITIZING SUBSTANCE IS NOT FREE IN THE BLOOD SERUM. 
The following work seems to indicate that the substance which 
renders the guinea pigs susceptible to a second injection of horse 
serum is not free in the blood serum of the guinea pig in the same 
sense that the free receptors are in diphtheria antitoxic serum: 
G. P. No. 298 (normal). Eight c. c. of a mixture containing 4 c. c. serum of susceptible 
guinea pigs and 4 c. c. normal horse serum, injected into the peritoneal cavity; the 
serums were mixed and injected at once. No symptoms. 
G. P. No. 299. Ten c. c. above mixture injected into the peritoneal cavity. No symptoms. 
G. P. No. 297. Five c. c. above mixture injected into the peritoneal cavity. No symptoms. 
From the fact that the sensitizing substance does not appear to be 
free in the blood of the guinea pig and from the further fact thatwe were 
unable to transfer the immunity we might infer that this reaction may 
not reside so much in the blood serum as in the cells of the body. 
THE EFFECT OF HEAT UPON THE SENSITIZING SUBSTANCE. 
The following experiments plainly prove that the sensitizing sub- 
stance is very resistant to heat, for guinea pigs previously treated 
with small quantities of horse serum heated to 60° C. for six hours 
subsequently proved highly susceptible: 
G. P. No. 265. Six c. c. normal horse serum (roan) injected into the peritoneal cavity. 
Dead in 38 minutes. 
[Previous treatment- 17 days prior, inoculated subcutaneously with c * c - normal 
horse serum (roan) heated to 60° C. for 6 hours.] 
G. P. No. 266. Do. Dead in 20 minutes. 
G. P. No. 267. Do. Dead in 25 minutes. 
