ACQUIRED IMMUNITY 125 



toxin had been made. He argued thence that antitoxin 

 must be a substance produced by the horse, and not merely 

 an altered toxin, for were it the latter the second lot of serum, 

 as containing less antitoxin, should be weaker than the first. 

 But as a fact his observations tell against the theory of 

 chemical neutralization, not against that of a vital reaction. 

 Definite quantities of chemicals are required for the neutral- 

 ization of other chemicals. But for the purpose of a vital 

 reaction the quality of the reagent is, within limits, of more 

 importance that the quantity as in the case just cited of 

 antitoxin from a single vaccination mark, which confers 

 complete, though not lasting, immunity. 



204 (a). The theory of immunity at present most in favour 

 is Ehrlich's " side-chain " hypothesis, a chemico-physiological 

 attempt to explain the facts. Ehrlich supposes that the 

 cells of the body have certain " outlying " groups of mole- 

 cules, which he terms side-chains or receptors, and which are 

 capable of union with various substances which may be pre- 

 sent in the blood ; for example, food or toxin molecules. By 

 means of these side-chains, therefore, cells are nourished or 

 on occasion poisoned. If a toxin be very abundant, in 

 the circulation, its molecules, seizing on many side-chains, 

 may become part of the cell and so cause its death. If less 

 abundant it causes the death of some of the side-chains 

 merely. Each of these, forming a compound with a now 

 inert toxin molecule, is shed from the cell and undergoes 

 disintegration. This is the chemical part of the theory. 

 The physiological part is that toxins, if they do not destroy 

 the cells, stimulate them to produce many side-chains, which 

 are shed in an active condition and serve to neutralize the 

 toxins when free in the lymph. For this reason the serum 

 of an individual who has had experience of the toxins possesses 

 antitoxic powers. When immunity is inborn it is supposed 

 that the cells possess no side-chains capable of uniting with 

 the toxins. They are therefore uninjured, a wonderful 

 difference thus existing between species of animals, and, 

 since some individuals possess inborn immunity to diseases 

 to which other individuals of the same species are susceptible, 

 even between individuals of the same race. 



204 (&). It is evident, if Ehrlich's hypothesis be correct, 

 that it does not controvert the explanation upheld by the 

 present writer. It merely attempts to go much deeper. For 

 digestion by enzymes it substitutes neutralization with sub- 

 sequent digestion. It explains " habituation " by supposing 

 that the cells acquire the power of producing side-chains 



