200 PARASITISM 



the new responsive bodies are called anti-bodies. Now the 

 facts of immunity are established and there is an increasing 

 multitude of such facts, but the explanations are purely theo- 

 retical. There are two main hypotheses at the present time ; one 

 is Metschnikoff 's development of phagocytosis, the other is Ehr- 

 lich'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 two kinds microphages 

 and macrophages. The former are the leucocytes of the blood, 

 which engulf bacteria and other minute bodies, and digest them 

 by the aid of an enzyme clled microcytase. The latter are modi- 

 fied organ cells of the body, which have become dissociated 

 from their tissues, and roam about as scavengers in the blood 

 supply, producing an enzyme called macrocytase, and digesting 

 larger bodies than bacteria. 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 unsatisfied 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 may unite with molecules of poison, 

 which are thus introduced into the protoplasmic substance, 

 resulting in the destruction of the side-chains. 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. Its blood, however, becomes loaded with 

 anti-bodies. According to Ehrlich's theory, this is explained 

 by the assumption that atom groups of the molecules in proto- 

 plasm, when thus destroyed by the poison molecules, are regen- 

 erated, and these regenerated groups are cast off from the proto- 



