392 HUMAN PHYSIOLOGY 



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 in- 

 creased 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 is injected into an animal which afterwards receives 

 a dose of the toxin, that toxin will not act, and the animal 



will be immune. 



Typhoid Toxin. But immu- 

 nity 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 



FIG 154.-TO illustmte the forma- of i ncreas i ng doses produce a 

 tion of Side Cha:ns, SC, by . . 



which the toxin molecules, T, serum which has the power of 



are either anchored to the cell destroying the Organism when 



:Lrtt7"an?- added to it even outside the 

 toxin is formed. 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 unimmunised animal be added, 

 the power is restored. Obviously the anti-body which 

 destroys the organism the bacteriocidal or bacteriolytic 

 body requires the co-operation of another body to enable 

 it to act, and this body has been called the complement. 

 Ehrlich supposes that the immune body does link to the 

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

 linked to the complement. The figure may help to explain 

 this (Fig. 155). 



Cytotoxins. Similar anti-bodies, acting upon the cells 

 of the animal body, may be produced by injecting 

 the particular kind of cell into an animal of another 

 species. Thus, if human blood be repeatedly injected 



