is that recovery from infection consists in the bringing into being of a new set of phenomena that gradually reinforce the resistance; that recovery from infection is accomplished through a process of immunization. The evidences of this condition of immunization are found in the appearance in the blood some time between the fourth or fifth to the tenth day of the disease, and somewhat later than they have appeared in the spleen and bone marrow, of chemical substances which are directed in a specific manner to the neutralization of the poisons having been and still being produced by the bacterial causes of the disease, to the destruction of the bacteria themselves either outright by the plasmatic fluid which has now been enriched by a new quantity of intermediary substance of high potency that may bring the bacteria more readily under the dissolving influence of the complement, or by the phagocytes to which they are exposed in greater measure through the production of opsonins of higher strength and stability. As recovery progresses these immunity substances continue to increase until at the termination of the disease they are present in quantities that suffice often, by a passive transfer to another individual, to protect other animals more certainly from an infection, or to terminate abruptly an infection already established in them.
When the infectious disease is the expression not of the combined effects of poison and bacteria but of the poison chiefly which enters the blood, the bacteria remaining without as in diphtheria, then the blood changes characterizing the immune state are simpler and consist in the accumulation there of antitoxins that constitute the most perfect antidote to poisons that are known. The condition of immunity produces no demonstrable change in the properties of the phagocytes through which they are better enabled to overcome the poisonous bacteria. They do become, in course of the immunization, more sensitive to positive chemotactic stimuli; but it is still an unsettled question whether they are altered qualitatively by the immunization, or whether the plasmatic changes do not really react upon them and thus increase their efficiency.
It must now be patent that between what may be termed the process of physiological resistance and what is termed the condition of immunization, a wide distinction exists. The one is non-specific in its action, the other highly specific in its effects; the one is subject to a limited augmentation, the other may be carried to a high degree of potency and perfection; the one often fails to protect the organism in which it is developed, the other suffices to protect both itself and another organism. If therefore we were to be asked in what manner can the animal organism best be reinforced against infection, we should be compelled to answer by passing safely through the infection itself. This conclusion, which has been reached by purely experimental biological methods is supported on every side by common observation and experience with