CHAPTER XIV. 



IMMUNITY 



Whether the study of biology be taken up by those who intend 

 practicing medicine, or for general cultural purposes, the fact remains 

 that all of us, at some time in our lives, require the se vices 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 in- 

 telligently follow at least the ordinary scientific articles appearing in 

 various magazines and journals written for educated men and women, 

 there has been some radical defect in our instruction. 



In view of the fact that practically all modern medicine is based 

 upon the theory of immunity, neither the medical man, the medical stu- 

 dent, nor the educated man at large, can intelligently discuss or intelli- 

 gently understand anything that muy 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, in fact, probably one 

 of the most difficult that 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. 



As all coelomates have their bodies arranged as one tube lying 

 within another, 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 atmosphere and environmental conditions that the 

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

 <iiverticula is really outside of the body in so far as the atmospheric sur- 

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

 intestinal-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 that 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 world), and 



