2 INTRODUCTION 



the causes are fully known ; the diseases themselves can be 

 reproduced (in most cases) at pleasure, and the physiological dis- 

 turbances which take place are fairly well investigated. There 

 are, of course, gaps, and those not inconsiderable, in our know- 

 ledge ; but, on the whole, the nature of these diseases is nearly as 

 well ascertained as the present state of normal physiology will 

 allow. Further, we can not only reproduce the diseases, but we 

 can reproduce in most cases any degree of immunity to them 

 which we may require for purposes of protection or research, and 

 we can investigate the differences between the cells and fluids of 

 the immunized person or animal and the corresponding parts of a 

 normal organism, and we can attempt to correlate them with the 

 production of the immune state. We have, therefore, a very 

 large amount of information on the subject, and although this 

 information is at present incomplete, we have already obtained 

 results of the highest practical and theoretical importance ; and 

 the value of these results leads us to believe with confidence that 

 our methods are right, that we are on the right track, and that a 

 solution of the problems that have at present baffled research will 

 come in the near future. 



As defined above, immunity is a function of all living material, 

 and one of the highest importance. Biologists have compiled 

 lists of the essential properties of living protoplasm nutrition, 

 reproduction, and the like but have not realized that immunity 

 to bacterial action is the first necessity for continued life. 

 Consider for a moment a small water animal say a hydra 

 occurring in water which naturally contains saprophytic bacteria. 

 Whilst the animal lives these organisms do not affect its proto- 

 plasm in any way, the latter being immune to their action ; but 

 on the animal's death rapid putrefaction occurs, and in a few 

 hours its protoplasm is broken down by bacterial action: the 

 immunity has ceased. Immunity to putrefactive bacteria is 

 therefore a condition of life in the lower animals. But the same 

 is true in every respect for those of a higher grade, man included. 

 From the moment of birth we are surrounded with air containing 

 bacteria which are not pathogenic in the ordinary sense, but 

 which only fail to be so because of the inherent power of immunity 

 to saprophytic bacteria, which is a fundamental property of all 

 living material. Apart from this, the organisms present in the 

 air, alimentary canal, skin, etc., would flourish as rapidly as they 

 do in a corpse, and life would only be possible for a few hours, 



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