THEORIES OF IMMUNITY 



21 



tions between cells and bacterial poisons. When the cells 

 do not succumb in the struggle they become more resistant 

 to the action of the same poison. But the presence and the 

 destruction of bacteria in the organism may provoke reactions 

 of still another nature. The poisons which they excrete are 

 not the only substances of which the protoplasm and the 

 membrane of the bacterial body are composed. There are 

 albumins which without being poisonous are not harmless 

 for the organism. These albuminous substances themselves 

 must be transformed in order to be assimilated and there 

 may result from them disturbances which we will discuss 

 later when we come to treat of pathologic states provoked 

 by the injection of albumin. It is sufficient to note here 

 provisionally that we have to consider an intravascular 

 immunity which may be explained in many cases by the 

 phagocytic theory of Metchnikoff as well as a cellular immun- 

 ity which is not so explained and which is the object of the 

 biochemical theory formulated by Ehrlich. 



Ehrlich's theory has for its basis, as we have seen, the 

 general conception that a reaction between two substances 

 is possible and can be determined only by chemical affinities 

 between these substances: and it would seem evident that a 

 body which is insoluble in the fluids of the organism can 

 exercise only a purely mechanical action on the fluids and 

 the tissues. Not being able to form any combination, it 

 cannot produce a reaction or any disturbance in the existing 

 equilibrium. 



Ehrlich pictures the process of immunization in the follow- 

 ing manner: a toxin is attracted and fixed by a cellular sub- 

 stance which possesses a chemical affinity for it: the toxin 

 neutralizes this substance and is itself neutralized. The 

 neutral product thus formed is expelled from the cell as use- 

 less and as the cell cannot be permanently deprived of the 

 substance which the toxin has neutralized and which we will 

 call normal antibody, it reproduces not only as much as is lost, 

 but a little more; it is the custom of every living cell to react 

 to any sort of excitation by a multiplication of the substance 

 excited, or, in other words, by a reinforcement of the tis- 

 sues and of the organs excited in order to be able to support 



