ADJUSTMENT TO INFECTIOUS DISEASE 4I3 



in the serum, which has a specific capacity for union with the 

 particular bacteria and which, by uniting with them, changes 

 them so that they are more easily destroyed and removed. 



There are, of course, many other factors involved, but for 

 the purpose of making thoroughly clear the remarkable 

 capacity of the body to adjust itself to an abnormal condition 

 which threatens its destruction, it is best to follow only the 

 main Hnes of occurrence, rather than to confuse the primary 

 issue by an abundance of less important detail. This dormant 

 capacity of the body to meet specifically an abnormal 

 condition which threatens its survival would be difficult to 

 understand if the described train of events were confined to 

 infection. But, although the "antibody" mechanism was 

 discovered first in connection with infectious diseases and, 

 in this relation, has its most immediate practical interest, 

 it is important to realize that this inherent capacity of 

 specific response applies broadly to the entrance of a multi- 

 tude of extraneous materials, of which bacteria are only 

 a small and relatively unusual class. To make this clear 

 it will be necessary to consider the processes that go on in the 

 metabolism of higher animals and the chemical nature of the 

 substances which normally penetrate within the physiological 

 interior of the body. 



In the lower forms of animal life digestion is intracellular, 

 and within specialized vacuoles solid particles of the foreign 

 substances are broken down into forms in which they can 

 be incorporated into the protoplasm of the cell. In the lower 

 metazoa the digestive process remains intracellular, but is 

 gradually being relegated to special endothelial cells. Through- 

 out the upward scale of the animal kingdom there is a gradual 

 substitution of extracellular for intracellular digestion. 

 In the higher forms of animal life normal digestion is so exclu- 

 sively the task of certain specialized intestinal enzymes 

 that the materials, which eventually enter the circulation 

 and are distributed to the cell units for assimilation, have 

 been converted into diffusible form, chemically adjusted 

 to cellular needs. Thus, it is likely that in the completely 

 normal body, a condition which probably never exists except 

 for short periods, no foreign fats, proteins or complex car- 

 bohydrates penetrate into the circulation, the fats being split 



