686 COLLECTED STUDIES IN IMMUNITY. 



existence of analogous groups for the assimilation of less complex 

 substances. So far as the simplest additional function is concerned, 

 namely, the absorption of oxygen, I believe this question is already 

 partly answered. It is well established that in the hsemogiobin 

 molecule it is exclusively the organically bound iron residue which 

 effects the loose union with the oxygen on the one hand, and the 

 carbon dioxide and hydrocyanic acid on the other. It will therefore 

 be necessary to assume that the red blood corpuscles contain 

 definite groupings which possess a maximum affinity for iron and 

 with that form a complex combination having the characteristic 

 functional properties. The protoplasm of the red blood corpuscles 

 would thus be characterized by a plentiful supply of " f erro-recep- 

 tors," the completing of which receptors with iron leads to the 

 finished hsemogiobin molecule. Similarly we shall have to assume 

 the existence of " cupri-receptors " in the blue respiratory pigment 

 of crabs, and perhaps of " mangano-receptors " in other animals. 

 The localization of iodine in certain glands, especially in the thyroid 

 gland, and also the fact that the iodine is associated with certain 

 aromatic side chains, will also be interpreted according to this 

 conception. 



The question as to whether the cell contains preformed chemo- 

 receptors for the great host of true therapeutic substances is one of 

 great difficulty. This leads us into the important domain governing 

 the relation between chemical constitution and pharmacological 

 action, which in turn constitutes the basis for the rational develop- 

 ment of therapeutics. Not until we have really learned the site of 

 attack of the parasites, when we have come to know what I term 

 the therapeutic biology of the parasites, will we wage successful 

 warfare against the producers of infection. 



For this reason I have begun studying the existence of 

 particular chemo-receptors on unicellular organisms, because here 

 the conditions are much more favorable for gaining a clear insight 

 than is the case in the extremely complex mechanism of the higher 

 organisms. The problem I undertook to solve was this: Do trypan- 

 osomes possess, in their protoplasm, definite groupings which bring 

 about the anchoring of certain particular chemical substances? 



If any particular substance possesses the power to kill trypano- 

 somes or other parasites in a test tube or in the animal body, it is 

 obvious that this can only be due to the fact that the substance is 

 taken up by the parasites. This bald fact, however, does not by 



