CHAPTER XIV 



IMMUNITY 



WHETHER the study of Biology be taken up by those who intend 

 to practice medicine, or for general cultural purposes, the fact 

 remains that all of us, at some time in our lives, require the 

 services of a medical man. Likewise, all of us, who make any pretense 

 whatever at being college men and women, feel, and rightfully so, that 

 unless we can intelligently follow at least ordinary scientific articles in 

 the various magazines and journals published for educated men and 

 women, there has been some radical defect in our instruction. 



Practically all modern medicine is based upon the theory of 

 immunity, and neither the medical man, the medical student, nor the 

 educated man at large, can intelligently discuss or intelligently under- 

 stand anything that may be told him regarding himself or the method 

 of treatment suggested when disease comes to him, unless the theory 

 of immunity is understood. 



The subject of immunity is rather difficult. It is, in fact, probably 

 one of the most difficult which confronts the first and second year student 

 of Biology; but his ability to grasp and understand the theory is, in a 

 way, a test of his ability at understanding and applying the knowledge 

 he has gained in Biology. 



All coelomates have their bodies arranged as one tube lying within 

 another, and, if one could draw out any coelomate body lengthwise, 

 the outer part would appear as a tube with very thick walls, while the 

 gastro-intestinal-tract would form an opening through the entire body. 

 In fact, the whole body would appear quite similar to an ordinary thick- 

 ened gas pipe (Fig. 164). 



One can readily understand that*the opening in the gas pipe is really 

 subject to the same atmospheric and environmental conditions that the 

 outside of the pipe may be. So, too, the intestinal tract, with all its 

 diverticula, is really outside of the body in so far as the atmospheric 

 surroundings are concerned. In fact, the interior portion of the gastro- 

 intesinal-tract is just as much outside the body (although not quite as 

 much exposed) as is the skin on the outer surface. 



Now, the surface on the inner side of the gastro-intestinal-tract, just 

 as the skin, forms a layer which can, under certain conditions, be pene- 

 trated by either physical, chemical, or living substances. We know that 

 we can scratch or cut ourselves. This results in a physical injury. We 

 know that poisons are chemicals which can injure tissues whenever 

 such poisons get into the system, and we also know that living organ- 

 isms, such as bacteria (unicellular organisms from the plant worlci) and 



