That the body is capable of reacting to parenterally introduced antigenic substances has been amply proven, but as yet there has been no satisfactory explanation of the mechanism of the interaction, that is, it has not been conclusively determined whether antigen is eliminated as antigen or whether it enters into the metabolic processes and becomes utilized or denatured. It can not be assumed that identical processes follow the introduction of different antigens or even of the same antigen when injected by different routes. In fact, opinion differs very widely as to what happens when a given antigen is introduced by a single route. Further complications arise through the fact that animals receiving antigen may, as a result of inherent peculiarities or experimental procedure, be endowed with a variety of metabolic forces, which in individual cases may so combine as to give a variety of results.

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