ADJUSTMENT TO INFECTIOUS DISEASE 4 II 



hood in which they are lodged, as well as remote areas to 

 which their toxic products are carried. The body responds 

 by a process which, in its main Hnes of strategy, includes 

 both a neutrahzation of the bacterial poisons and a destruc- 

 tion of the invading cells by the blood plasma and by the 

 phagocytic action of the white blood cells and of certain 

 fixed tissue cells. If the body survives, a subsequent invasion 

 of the same bacteria encounters a considerable enhance- 

 ment of all these properties, which results in a much more 

 rapid disposal of the invaders. When we say that the reaction 

 is "specific," we approach the most mysterious biological 

 fact in the process, and mean thereby that this entire train 

 of events is strengthened by the first infection only in regard 

 to the same or to closely related infectious agents. 



The simple observation that a body which has survived an 

 infection is thereafter resistant to reinfection by the same 

 agent for periods from fractions of a year to the remainder 

 of life, is as old as history, was known in ancient China and 

 India, was recorded by Thucydides in regard to the plague 

 in Athens, and was generally familiar to clinicians when 

 Jenner applied it experimentally in smallpox. It was scien- 

 tifically formulated by Pasteur with bacteria, and its analysis 

 has constituted the material of the science of immunology. 



Briefly summarized, the basic facts of the analysis, as far 

 as it has gone, are as follows: Normal animals, whether they 

 possess a demonstrable degree of resistance or no resistance 

 whatever to a given infectious agent, may be rendered 

 highly resistant, sometimes even completely immune, by a 

 variety of methods. These are: 



1. The survival from an infection with the particular micro- 

 organism administered either in small doses or in an attenuated 

 (weakened) form. 



2. Systematic dosage with the dead organisms. 



3. Similar treatment with the products of the organism (toxins). 



When the resistance has been achieved, the animal has 

 changed profoundly in its reaction to this particular infection 

 and to no other, and its immunity can to a large extent be 

 referred to the blood plasma and to certain special cells. 

 If, as in diphtheria or tetanus, the toxins are the important 



