196 PARASITISM 



opment of phagocytosis, the other is Ehrlich's famous side chain 

 hypothesis. According to the former all responses of anti-body 

 formation to antigens take place in the phagocytes of which 

 there are twX> kinds microphages and macrophages. These 

 are modified organ cells of the body which have become dis- 

 sociated from their tissues and roam about as scavengers in the 

 blood supply. When broken down under the action of antigens 

 they liberate chemical substances which form the counteracting 

 chemical anti-body. The reaction thus is purely chemical or 

 physiologico-chemical in nature. 



In Ehrlich's theory there is an attempt to visualize the actual 

 process of the physiologico-chemical action. The unknown 

 myriads of molecules which make up protoplasm are imagined 

 to have free groups of atoms ready to unite with food substances 

 or other substances from the blood. These free groups are 

 called side chains. Instead of uniting with food substances one 

 or many unite with molecules of poison which are thus intro- 

 duced into the protoplasmic substance resulting in its destruc- 

 tion. If the number of such molecules of poison is limited, the 

 protoplasm is able to regenerate the atom groups thus used, 

 but if the poison accumulates the new groups are combined 

 as soon as formed and fatal poisoning results. In the case of 

 diphtheria the horse undergoes such direct poisoning but not 

 extensive enough to produce fatal results. The blood, however, 

 becomes loaded with anti-bodies. According to Ehrlich's 

 theory the atom groups of the molecules in protoplasm when 

 united with poison molecules are regenerated and these regen- 

 erated groups are cast off into the blood as free atom groups, 

 capable of uniting in the blood with the poison molecules. In 

 this way chemical union of antigen and antibody takes place 

 outside or apart from protoplasm and when united the poison is 

 made harmless because of its inability now to unite with any 

 protoplasmic group its valencies have been satisfied. Fur- 

 thermore, Weigert has shown that the regeneration of free atom 

 groups in the blood (anti-bodies) is out of all proportion to the 

 toxin which stimulated the regeneration. In other words, 

 hyper-regeneration follows such toxic injuries to the protoplasm. 



