492 IMMUNITY. 



a source of antitoxin. Cells, to whose disturbance, resulting- 

 from the fixation of toxin, characteristic symptoms of poisoning 

 are due, will thus be sources of antitoxin, e.g. cells of the nervous 

 system in the case of tetanus, though the cells not so seriously 

 affected by toxin fixation may act in the same way. 



When we come to consider how far Ehrlich's theory is in 

 harmony with known facts, we find that there is much in its 

 favour. In the first place, it explains the difference between 

 active and passive immunity, e.g. difference in duration, etc. ; in 

 the former the cells have acquired the habit of discharging anti- 

 substances, in the latter the anti-substances are simply present 

 as the result of direct transference. It is also in harmony with 

 the action of antitoxins, etc., as detailed above, and especially it 

 affords an explanation of the multiplicity of anti-substances. 

 For, if we take the case of antitoxins, we see that this depends 

 upon the combining affinity of the toxin for certain of the cells 

 of the body, and this again is referred back to the complicated 

 constitution of living protoplasm. Furthermore, the biological 

 principle involved is no new one, being simply that of over- 

 regeneration after loss. 



It is to be noted, however, that it does not explain active 

 immunity apart from the presence of anti-substances in the 

 serum. For example, an animal may be able to withstand a 

 much larger amount of toxin than could be neutralised by the 

 total amount of antitoxin in its serum. It is difficult to see 

 what condition of the receptors of the cells would explain such 

 a fact, and the question arises whether there may not be really 

 an increased resistance of the cells to the toxophorous affinities. 

 Further, when the serum of an animal contains a large amount 

 of antitoxin, how does the toxin reach the cells in order to influ- 

 ence them as we know it does ? This is difficult to understand 

 unless the toxin has a greater affinity for the receptors in the 

 cells than for the free receptors (antitoxin) in the serum. A 

 supersensitiveness of the nerve cells of an animal to tetanus 

 toxin, sometimes observed even when there is a large amount of 

 antitoxin in the serum, has been often brought forward as an 

 objection. But this also may perhaps be explained by there 

 having occurred a partial damage of the cell protoplasm by the 

 toxophorous action in the process of immunisation an expla- 

 nation which, of course, demands that in some way the freshly 



