By these experiments it has been possible to show definitely that the natural immunity of rats for diphtheria toxin and chickens for tetanus is due to two factors—a special resistance of at least certain of the cells of these animals and further to the existence of neutralizing substances in their plasma. The plasma of these animals protects not only the cells of these animals against lethal doses of toxin but it also protects the cells of susceptible animals.

So far heart muscle and ovary or in other words connective tissue cells have alone been studied. The resistance of many other tissue may be similarly investigated; such as, nervous tissue, other epithelial tissues and lymphoid tissue.

A sufficient number of experiments have not been made to give quantitative results. The method allows this study and a further investigation into the nature of the cellular resistance and the mechanism of neutralization as indicated in the plasma tests.

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