Some of the theories which have been advanced to explain immunity from infectious diseases have a bearing upon the production of antitoxin; others leave this out of consideration. It is evident that no theory can stand if it offers no explanation of the reason why it is possible to use the blood serum of inoculated animals to protect and cure other animals and human beings. It will be my purpose in this paper to put the various theories, that have been advanced, to this test. Some of the theories merely attempt to explain why the bacteria do not grow in the body of non-susceptible animals and leave out of account the neutralization of the disease producing products, and for this reason fail to be universally applicable.
1. The exhaustion theory of Pasteur claims that the bacteria are unable to grow in the body of an animal that has suffered from