IMMUNITY 125 



In 1894 Pfeiffer discovered that cholera spirilla introduced into 

 the peritoneal cavity of a guinea pig highly immunized against 

 them lost their motility almost immediately, gradually became 

 swollen and granular, and finally passed into complete solution. 

 These changes are generally spoken of as " Pfeiffer's phenome- 

 non," or bacteriolysis. 



As a result of these and many other observations Ehrlich offered 

 an explanation based on a theory advanced some years before 

 to account for the process of cell nutrition. He thought of the 

 cell as possessing two functions: one physiological, such as that 

 of a gland cell to secrete, and the second function one of nutrition 

 concerned mainly in nourishing and repairing the cell. More- 

 over he attributed this double function to each molecule com- 

 posing the complex cell. That portion of the molecule by means 

 of which it secures nourishment is of greatest importance in rela- 

 tion to immunity. Ehrlich pictured this molecule of protoplasm 

 as a functional center with a large number of side chains or recep- 

 tions, each of these side chains possessing the ability to select 

 the food atom it needs from the surrounding blood and lymph, 

 to combine with it, and by a kind of absorptive process to incorpo- 

 rate it in the molecule. The food molecules are conceived as pos- 

 sessing a portion specially adapted for union with the side arm of 

 the cell, so that when the two are brought in relation they fit to- 

 gether in much the same way that a key fits a lock. As food mole- 

 cules vary in their chemical composition it is reasonable to assume 

 that the cell receptors prepared to anchor them differ : they may be 

 of a simple constitution and adapted only to the taking up of a 

 very simple substance or they may be more complex and able to 

 digest a larger food molecule. 



It is easy, then, to conceive that when bacteria and other foreign 

 substances are introduced into the body fluids certain receptors 

 may anchor them as they pass in just the same manner as a simi- 

 larly constituted food molecule. Combination with such sub- 

 stances, however, leads to a different result; instead of being 

 nourished the cell may be poisoned and in all probability lose the 

 receptor that had attached itself to the foreign substance. If, 



