the acute infectious diseases of which one attack protects from a subsequent attack of the same disease.
It may conduce to clearness if we should enumerate the factors that have been described and assigned on the one hand to natural resistance, and on the other hand to acquired resistance or immunity. We can tabulate the factors in the following manner:
This tabulation exhibits the distinction between the physical basis of physiological resistance and of the state of immunity. There is another difference between them; any increase that can be called out beyond the mean of physiological resistance is accomplished in a few hours; and having been called out to meet a particular condition of need of the body and the effect having been exerted, it passes off very soon. It is rare that the effect of a hyperleucocytosis can be detected for more than three or four days after it has appeared. The development of the state of immunity, on the other hand, is a slow process relatively and depends upon the setting into motion of certain cell functions, through which new substances are produced, which, being first retained within the cells producing them, eventually are passed into the blood. Hence it is that these new substances can be detected at an earlier period of the infection in the spleen than in the blood. But once they have been produced, the substances endure either for an indefinite period, or the capacity to produce new ones of the same sort is retained by the organism often for years. The blood may grow weak in the typhoid immunity principle in the course of years following an attack of typhoid fever, or a rabbit immunized with typhoid bacilli may show after a time a great diminution of the blood agglutinins for typhoid bacilli; but the typhoid immunity persists in the one, as in the other-minimal quantities of typhoid bacilli will bring out, and without the original delay, a new production of agglutinin that will restore the lost amount.