ADJUSTMENT TO INFECTIOUS DISEASE 423 



though given in large amounts. This latent reaction capacity 

 with or without formation of antibodies is a mystery. 

 Its recognition has explained many abnormal conditions 

 in man and has led to much exact analysis, but it has 

 remained in its fundaments utterly unexplained. 



In order to obtain a proper physiological understanding 

 of infectious processes, therefore, it is simplest to remember 

 that the body cells of bacteria are composed of protein 

 materials, largely nucIeoprotein-Hke substances, which pos- 

 sess this antigenic function. As soon as bacteria that have 

 become biologically adapted to entrance and survival in 

 the animal body have invaded either through the skin, the 

 respiratory or intestinal tracts, and have penetrated into 

 the physiological interior they become foreign antigenic 

 substances in the same sense in which this would apply to 

 egg white or horse serum which had been experimentally 

 injected. In the case of bacteria, however, the antigen is a 

 living cell which, because of its development of parasitic 

 properties, is capable of increasing in quantity at the expense 

 of the host. Also, these bacterial invaders often produce 

 poison which, in different bacteria, possess varying selective 

 pharmacological affinities for definite parts of the body; 

 and these poisons often are in themselves antigens. The 

 nature of the disease, therefore, depends upon the manner 

 of entrance, the amount of local inflammation aroused at 

 the point of entrance, the distribution of the organisms in the 

 body and the particular tissues of the host which are selec- 

 tively injured by the poisons. Upon these factors depend the 

 manner of infection, the incubation period and the nature of 

 the symptoms; and if we know what the biological prop- 

 erties of the various bacteria are in these respects, we 

 have a logical basis for diagnosis and can often state by 

 which particular microorganism or type of microorganism 

 the disease is caused. 



The antigenic substances which are liberated from the 

 bacteria and come in contact with the cells give rise to the 

 specific increase of reaction capacity for this particular 

 antigen. In some cases this, let us call it "increased specific 

 irritability," remains purely a cellular function and may be 



