412 



VETEKINARY PHYSIOLOGY 



special affinity (fig. 166). He supposes that some of these 

 side-chains fit the toxin molecule, and are thus capable of 

 anchoring it to the cell and allowing it to exercise its toxic 

 action, and he explains the production of antitoxin by supposing 

 that, as these side-chains get linked to the toxin and are thus, 

 as it were, thrown out of action, others are produced to take 

 their place, since they are necessary for the nourishment of 

 the protoplasm. If the toxin is continually administered in 

 small doses this production of side-chains may be so increased 

 that they get thrown off into the blood and in it are capable 

 of linking to the toxin and so preventing it from fixing itself 

 to the cells. If, therefore, some of the blood be injected into 



SC 



FlG.166. To illustrate the forma- 

 tion of side-chains or receptors, 

 sc, by which the toxin mole- 

 cules, T, are either anchored 

 to the cell or neutralised. 

 When the side-chains are set 

 free an anti-toxin is formed. 



FIG. 167. To illustrate the 

 anchoring of the anti-body 

 or amboceptor, , to the 

 cell by a side-chain or re- 

 ceptor, sc, and the action 

 upon it of complement, 



an animal which afterwards receives a dose of the toxin, that 

 toxin will not act, and the animal will be immune. 



Typhoid Toxin. But immunity may also be established not 

 against toxins separate from organisms, but against organisms 

 which hold their toxin, as in the case of the bacillus of typhoid 

 fever. Here repeated injections of increasing doses produce 

 a serum which has the power of destroying the organism when 

 added to it even outside the body. "But this is not a simple 

 combination, because if the serum be heated to 55 C. it loses 

 its power, but if a few drops of the fresh serum of an unim- 

 inunised animal be added, the power is restored. Obviously 

 the anti-body which destroys the organism the bactericidal 



