734 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
The facts at our command point to the great potential power of 
the normal organism to resist infection and indicate that the normal 
body possesses the capacity, on demand, to increase this power beyond 
the mean value, chiefly by opposing intending infection by hyper- 
leucocytosis and also, probably, by the strengthening of its plasmatie 
defensive action through the additional soluble alexin substances 
thrown off by the augmented leucocytes. This defensive mechanism 
acts in the same manner on all bacterial invaders and is not specially 
adapted for any one or group of bacteria. The form of activity is 
strictly nonspecific. 
Let us now ask ourselves if in overcoming infectious disease, which 
luckily the organism is frequently able to accomplish, the mechanism 
put into operation is similar and only more intense than the one we 
have considered for warding off infection. The answer to this ques- 
tion is that recovery from infection consists in the bringing into 
being of a new set of phenomena that gradually reenforce the resist- 
ance; 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 neutraliza- 
tion of the poisons having been and still being produced by the bac- 
terial causes of the disease, to the destruction of the bacteria them- 
selves either outright by the plasmatic fluid which has now been en- 
riched by a new quantity of intermediary substance of high potency 
that may bring the bacteria more readily under the dissolving in- 
fluence 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 im- 
munity 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 un- 
