More than fifty years ago, Pasteur and his co-workers thought that acquired immunity to specific infections might be due to the destruction of food factors in the body necessary for bacterial growth. This so-called specific exhaustion theory, however, was not substantiated. The chief objection to it was the failure to conceive of a specific antiserum capable of carrying a specific chemical deficiency to another animal.
The recent work of Schern and Artagavegtia-Allende1 of Montevideo brings this theory from the purely historical to a stage of possible practical importance. Schern observed some thirty years ago that trypanosomes in citrated blood samples from recently infected rats retained a high degree of motility for several hours at room temperature. The motility of these parasites, however, was much more poorly retained when they were isolated from the animals in the later stages of experimental infection. The parasites thus obtained lost all motility within