THEORIES OF iMMUNlfY 
123 
circulates in the blood stream; that is, before it can reach the cell itself. 
It will l)e seen, therefore, that the same mechanism of the living;: body 
which is susceptible to beinj^; poisoned becomes the protective agent if 
it circulates in the blood stream. It is obvious that the cast-off side- 
chains constitute antitoxin. The body as a whole is qualitatiAely the 
same after as before these side-chains are formed in excess of the normal 
cellular needs; the difference is a quantitative one. An animal is 
Fig. 7.— Side-chains, third order (bacteriolysins, hemolysins and cytolysins). 1, 
side-chain attached to cell; c, haptophore group; h, complementophile group; 2, side- 
chain to which is attached a bacterial cell (6) and complement (5) ; 3, a cast-off side- 
chain of the third order; amboceptor; 4, a cast-ofT side-chain to which are attached a 
bacterial cell (6) and complement (5) illustrating lysis; 5, complement. 
naturally immune to soluble toxin, according to this theory, if the cells 
of the body do not unite with the toxin, that is, if they do not contain 
side-chains which fit the toxin "as a key fits a lock," to use Emil 
Fischer's analogy. Toxin may circulate in the blood stream of such 
animals, but it does not unite with the cells. 
Side-chains of the First Order.— From the standpoint of the side- 
chain theory, the toxin molecules consist of two groups— a combining 
