There is little accurate knowledge concerning the fate of foreign protein introduced into the body. It appears in the blood and in some instances is excreted in relatively small quantity in the urine but this method of elimination offers no satisfactory explanation of its long continued persistence in the blood nor of its final disappearance. Those who have studied the fate of protein introduced into immunized animals have found that the antigen enters the blood and some have observed no difference in rapidity of disappearance between the normal and the immune animal. The subject is one of broad interest for there is no line of separation between relatively bland substances such as horse serum or egg white on the one hand, and on the other hand toxic substances such as the serum of lower vertebrate and of invertebrate species and other products of the animal or vegetable kingdom.

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