CHAPTER IX. 



IMMTJNITY. 



It is a matter of observation as old as the history of man that all 

 animals are not alike affected hj the same diseases, and that epi- 

 demics which decimate certain species are wholly without effect upon 

 the health and life of others. Primitive man undoubtedly recognized 

 the fact that one attack of certain diseases gives a more or less per- 

 manent immunity to the same disorder. It has also been long recog- 

 nized that certain diseases, such as diphtheria and scarlet fever, which 

 are so highly fatal among children, but rarely attack adults. The 

 question of immunity has long been, is, and probably wiU long con- 

 tinue to be one of the most perplexing problems which medical 

 science has attempted to solve. With so many chances of infection, 

 the question is often asked how is it that the human race continues 

 its existence, and why has it not been swept out of the world by 

 epidemics ? The answer to this question is not easy, and the time 

 has not come when it can be given with perfect satisfaction. How- 

 ever, we are now in possession of numerous facts bearing on this 

 point, and it will be our attempt in this chapter to briefly state the 

 most important of these. 



It should be plainly understood that the factors involved in secur- 

 ing immunity against infectious diseases are multiple in number and 

 varied in character. A mistake has been made in endeavoring to 

 explain immunity as being due to one, or even to a few anti-bac- 

 terial properties of the animal body. This has been admirably 

 pointed out by Meltzer,^ from whom we make the following quota- 

 tion : " I maintain in the first place that in the struggle against 

 bacteria the defense of the body is not carried on exclusively or 

 chiefly by a single element. It is neither the body fluids, nor the 

 leucocytes, nor the other cells alone which can claim the exclusive 

 merit of maintaining the health of the body, but each and every one 

 of them has its variable share in attaining the desired end. . . . 

 Let us take as an illustration the protection of the conjunctival sac. 

 It is nearly in direct contact with the air, and we might expect to 

 find there an extensive bacterial settlement. Nevertheless Lacho- 

 witz and Bujwid found that in 69 per cent, of cases the conjunctiva 

 was perfectly sterile. The factors which accomplish this sterility, 

 or at least comparative sterility, of bacteria are : The reflex which 

 causes the closure of the lids at the approach of dust (the carrier of 



1 Congress of Am. Physicians and Surgeons, 1900. 

 162 



