118 IMMUNITY AND INFECTION 



mon, tacitly at least, two important features: the specificity of the 

 protective substances (antibodies) formed as the result of infection, 

 and the principle that no new mechanism is evolved de now to meet 

 the conditions existing during an infection; rather, there is an increase 

 in activity along definite lines in the preexistent, latent or reserve 

 mechanism of defense. 



Neither theory affords a satisfactory explanation of all the features 

 of immunity following infection and it is very probable that cellular 



FIG. 5. Side-chains, first order (antitoxins and antif erments) . 1, side-chain attached 

 to cell; c, haptophore group; 2, side-chain to which is attached a toxin molecule; 3, 

 a cast off side-chain of the first order: antitoxin or antif erment; 4, a toxin or enzyme 

 molecule; a, toxophore group; b, haptophore group; 5, a toxoid: the toxophore group 

 is destroyed, leaving the haptophore group (6) intact; 6, a toxin molecule attached to 

 a cast-off side-chain (antitoxin), illustrating the neutralization of toxin by antitoxin in 

 the blood stream. 



activity and the production of specific antibodies is more important 

 in certain types of infection, while phagocytic activities are more 

 intimately concerned in other types. 



A. The Humoral, Side-chain or Ehrlich Theory of Immunity. 

 According to Ehrlich's conception, every cell of the body has two 

 functions: a physiological function, which constitutes a special type 

 of activity of the cell secretory for a glandular cell, contractile for 

 a muscle cell, or conductive for a nerve cell and a nutritional func- 

 tion, which is concerned with the removal of the necessary food sub- 

 stances from the general supply circulating in the blood or lymph 



