202 PUBLIC HEALTH BACTERIOLOGY 



i antitoxin unit, " limes zero," expressed as L ; and 

 the quantity required to neutralize i unit of antitoxin 

 and yet, on injection, kill the guinea-pig in 4 to 5 days, 

 " limes death, expressed as L +. Then, according to his 

 theory, if T represent i toxin unit, 



L = lOOxT and L+ = L + ioixT = 201 xT. 



Ehrlich's side-chain theory is based on his previous 

 researches on the oxygen requirements of the organism, 

 linked up with those on diphtheria antitoxin. Borrowing 

 the language of organic chemistry, he likens the highly 

 complex albuminous and other molecules of animal 

 nutrition, to those complex compounds of the aromatic 

 series which chemists have dissected into a central group, 

 in which the elements may be represented as a hexagon 

 or ring of the benzene type, and the various other parts as 

 side-processes or side-chains. Thus benzene has derivatives 

 like the following, the added groups of which may be 

 spoken of as side-chains. 



.CH-CR 



BENZENE CH< >CH 



X CH - CW 



X).CH 3 

 CH - C^ 



ACET. CH 3 .CH 2 .-C/ >C-CO.OCH 3 



EUGENOL X CH - ZW 



Applying the same ideas to living cells, Ehrlich believes 

 that these cells have side-chains which have certain special 

 affinities. In this way the diphtheria toxin may be 

 supposed to be bound to certain nerve cells ; and likewise 

 tetanus toxin. The side-chains he calls receptors. When 

 thus bound by a toxin molecule, they are supposed to be 

 useless to the cell and are cast off into the blood-stream, 

 and the cell is supposed to be stimulated to produce more ; 

 not only so, but stimulated to produce an overplus which 

 is alleged to be then cast into the blood-stream, as the 

 cell would become overstocked. Thus he accounts for 

 the presence of antitoxin free in the blood, the free receptors 

 acting as antitoxin to the toxin circulating. The toxin, 

 which thus unites with the antitoxin, he conceives as 



