104 MAN --AN ADAPTIVE MECHANISM 



by the phagocytes. Other types of chemical defense 

 mechanisms, therefore, have been evolved to cope 

 with these more generally disseminated dangers. 

 Like the simpler process of leucocytosis, the action 

 of these more complex chemical mechanisms shows 

 a schematic resemblance to the process of digestion 

 in its more highly evolved stages. In sponges and 

 hydrse, for example, food is ingested by the individual 

 cells lining the gastric cavity, the food particles being 

 taken in directly, just as the leucocytes of the mammal 

 take in foreign particles, or the amoeba engulfs its 

 food. In higher organisms this intra-cellular process 

 of digestion has been succeeded by a process differenti- 

 ated among a number of specialized cells lining the 

 digestive tract, some of which secrete digestive fer- 

 ments which in turn prepare the food for absorption. 

 A similar modification seems to have taken place in 

 the means adopted by the organism for its defense 

 against infection. Thus, while certain of the leucocytes 

 destroy bacteria by a process of intracellular digestion, 

 other cells of the body seem to be excited by the in- 

 vasion of certain other microorganisms to secrete into 

 the surrounding body fluid chemical substances which 

 act as poisons to the bacteria or as neutralizers of the 

 poisonous secretions from the bacteria. 



This production of anti-bodies is in every instance 

 essentially a specific reaction excited by a specific 

 chemical stimulus. That is, the anti-bodies produced 

 in response to an invasion of diphtheria germs are 

 specific for that microorganism and useless as a defense 

 against the invasion of any other microbe. The 

 antitoxin of cerebro-spinal meningitis is powerless to 



