REGULATORS 613 



formed in the horse which seizes on the molecules of the toxin 

 and makes them harmless, just as when soda is added to sul- 

 phuric acid a neutral salt is formed. 



The two molecules have a definite chemical affinity for one 

 another, so that the toxin or antigen is no longer free to seize 

 upon the protoplasm of the animal's body. To explain this, 

 Ehrlich has suggested that the protoplasm molecule (fig. 233) 

 is made up of a central core with a number of side-chains or 

 receptors, which play an important part in taking up nourish- 

 ment of different kinds, special receptors being developed for 

 each kind of material. 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. The 

 production of antitoxin he explains by supposing that, as these 

 side-chains g-et linked to the toxin and are thus, as it were, 

 thrown out ot 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 they 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 an animal 

 which afterwards receives a dose of the toxin, that toxin will 

 not act, and the animal will be immune. 



2, Enteric Toxin. — But immunity may also be established 

 not merely against toxins separate from organisms, but against 

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

 of enteric fever. Here, repeated injections of increasing doses of 

 the dead bacilli cause the production of a serum which has the 

 power of destroying the organism when added to it even outside 

 the body. 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 even of an unimmunised animal be added, the power 

 is restored. Obviously the anti-body which destroys the organ- 

 ism^ — the hactericidal or bacteriolytic body, often called the ambo- 

 ceptor — requires the co-operation of another body to enable it 

 to act, and this body has been called the comiolement or alexine. 

 This is readily destroyed at a comparatively low temperature. 

 Ehrlich supposes that the immune body does link to the proto- 

 plasm of the organism, but that it must, in its turn, be linked 



