IMMUNITY. 



81 



ceptibihty to disease is a result of the antagonistic action of tlie 

 body Hinds (humoral hypothesis). Ehrlich's lateral chain the- 

 ory assumes that the cells of immune animals are not capable 

 of combining with the toxins of bacteria, i. e., they have no 

 receptor molecules and hence those animals are not receptive, 

 they are immune. Whether we accept the cellular hvpothesis! 

 the humoral hypothesis, or Ehrlich's lateral chain theory, the 

 fact remains that natural nnmunity is a characteristic or prop- 

 erty of parental origin that is transmitted to the oitspring and 

 is present at the time of birth. 



Natural immunity may be the result of an actiuired toler- 

 ance due to natural selection and heredity. There is a marked 

 variation in susceptibility and resistance in individuals of a given 

 species. A continuous or repeated exposure of susceptible ani- 

 mals to a given pathogenic microparasite will result either in 



Fij^. rjji. — I.\<jdeH ricinus, female, after (B. A. I.) 



destruction of those animals or the production of an iiumumtv. 

 i. e., those individuals least resistant will survive and their 

 resistance will become more and more fixed and will finally be 

 transmitted to the offsprmg and hence be a natural immunity. 

 Thus all native Cubans are practically immune to vellow fever 

 because at the time yellow fever was first introduced into Cuba 

 :he least resistant individuals died of the maladv, the most 

 resistant individuals survived and lived in the presence of the 

 diseases almost continually after yellow fever was introduced 

 into Cuba (It was not eliminated until after the Spanish-Amer- 

 ican war). Consequently the Cubans for several generations 

 developed in the midst of yellow fever and only the resistant 

 individuals survived. This resistance finally became so firm 



