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of the living plasma and cells of the normal animal. As the 

 animal's individuality is the resultant of all the bodily forces 

 at work, this equilibrium may be regarded as an expression of 

 that individuality. The equilibrium is disturbed when a new 

 factor is introduced, such as the presence of a foreign protein; 

 and the disturbance may result in the temporary or permanent 

 establishment of a new equilibrium. The new equilibrium makes 

 a difference in the animal's vital reactions, and this difference 

 may be particularly noticeable on re-introduction of the foreign 

 element which caused the original disturbance. The new 

 equilibrium, it is suggested, may be regarded as the real explana- 

 tion of a property (acquired immunity) which is more usually 

 attributed to a newly formed special substance (antibody) ; just 

 as natural immunity may be explained by the old equilibrium 

 rather than by a supply of natural antibodies. 



This view, though not sufficiently tangible to be regarded as 

 amounting to a theory of immunity, may be useful in two respects. 

 (1) It is certainly desirable to get rid of the word " habit," 

 which is meaningless when offered in explanation of biological 

 data. On the other hand, the conception of varying conditions 

 of chemico-physical equilibrium is based on experimental facts; 

 and, though these experiments do not reproduce the much more 

 complex interactions which take place in the living body, there 

 can be no doubt that questions of " equilibrium " are of high 

 importance in vivo. (2) In the desire for simplification, there 

 is a natural tendency to explain a result as due to the operation 

 of a single force, e.g., to the action of a particular substance 

 termed an antibody. But the real explanation may be much more 

 complex, owing to the large variety of forces or circumstances 

 making up the complete causal nexus of events to which the 

 result is really attributable. What has been said above about 

 colloidal equilibrium may be taken as an expression of this more 

 complex conception. 



This idea of equilibrium may also be correlated with another 

 way of attempting to explain a well recognised difficulty. In 

 the animal body there are many substances (ferments being the 

 commonest example) which are known to possess active 

 properties but do not exert their activity upon the living material 

 of the body. By way of explanation, it has been found necessary 

 to suppose that in the living animal there are various inhibitory 

 or antagonistic forces, acting as " buffers " or as " antienzymes " 

 or in other ways, and that these prevent the occurrence in vivo 

 of reactions which readily take place in vitro. This idea that 

 " anti " substances are part of the balanced mechanism of the 

 normal animal has found its way into discussions of some of the 

 obscure problems of immunity. I take the following example, 

 which is an endeavour to find some common ground for the 

 phenomena of natural and acquired immunity. It is based on 

 the hypothesis that all animals possess, in greater or less degree, 

 powers of resistance against any particular bacterium, and that 

 the differences between the highly susceptible, the moderately 



