89 
We believe that the substance which sensitizes the animal is iden- 
tical with that which later poisons it, absurd as that may seem at 
first glance. However, the first substance must cause a reaction in 
the organism resulting in the production of “antibodies’’ and it is 
these antibodies combining with a substance in the horse serum that 
produce the toxic action. We have found that small quantities of 
serum produce, after a definite period of incubation, a condition of 
anaphylaxis. Large quantities probably produce a lesser grade of 
hypersusceptibility. Multiple or repeated injections produce immu- 
nity. We therefore possess in horse serum a substance capabJe of 
causing both anaphylaxis and prophylaxis. 
