848 CHEMOTHERAPY 



these he terms chemoreceptors. While Ehrlich originally assumed that 

 the so-called side-arms of the protoplasmic molecule served primarily for 

 the process of nutrition, he now believes that these special chemore- 

 ceptors do exist. He suggests that they may probably possess a less 

 complex structure, similar to the receptors of the first order for simple 

 toxins, that they are more firmly attached to the cell, and that, accord- 

 ingly, they are less readily cast off, thus explaining why crystalline 

 chemical substances are, as a general rule, incapable of eliciting the pro- 

 duction of corresponding antibodies. This theory is based upon the 

 discovery that certain strains of a microparasite develop a state of 

 " resistance" or "fastness" to a particular substance, and that this ac- 

 quired characteristic may be transmitted from generation to generation. 

 This subject will be further discussed elsewhere. 



According to Ehrlich's postulate, therefore, toxic agents cannot act 

 on microorganisms unless they are fixed by suitable receptors (corpora 

 non agunt nisi fixata) . This conception is similar in every way to his 

 conception of the processes of infection and immunity and the develop- 

 ment of antibodies; that is, the toxic agent must first be " fixed" or 

 "anchored" to the molecule of a cell by suitable receptors by a process 

 of chemical interaction before damage can be inflicted. Chemoreceptors 

 differ from other receptors in being more firmly attached to cells, so 

 that while the cell may become immune to the toxic effects of the agent, 

 the blood-serum may not contain the immune bodies. 



When arsenic is introduced into the body, it is "fixed" by the re- 

 ceptors of certain cells; mercury in turn is fixed by other receptors, and 

 so on through the list. The basic principle of chemotherapy is that it is 

 possible to produce chemical substances that carry side-arms capable of being 

 fixed by microparasites, and to a much less extent by the body-cells. 



It is not necessary that the whole molecule of a toxic substance possess 

 a combining affinity for certain receptors: if one or more atom groups 

 becomes attached, it is presumed that it carries with it the remainder 

 of the molecule. Moreover, that atom group that is anchored or is re- 

 sponsible for the anchorage of the entire molecule need not possess any 

 of the properties of the entire molecule or of any part thereof. For ex- 

 ample, when the molecule of salvarsan is anchored to the spirochete of 

 syphilis by its OH or its NH 2 side-chain, or by both, the spirochete must 

 later contend with two molecules of arsenic, which, being in a trivalent 

 condition, can exercise its toxic effects to a marked degree upon the para- 

 site. The side-arms as they exist in salvarsan are much more readily 

 taken up by the spirochete than by the body-cells, which is shown by 



