1 6 THE POPULAR SCIENCE MONTHLY 



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 immuniza- 

 tion. 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 plas- 

 matic fluid which has now been enriched by a new quantity of inter- 

 mediary 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 ani- 

 mals more certainly from an infection, or to terminate abruptly an in- 

 fection 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 anti- 

 dote 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 posi- 

 tive 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 immuni- 

 zation, 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 conclu- 

 sion, which has been reached by purely experimental biological methods 

 is supported on every side by common observation and experience with 



