INFECTION AND IMMUNITY 



355 



other the interaction resulting in solution or cytolysis 

 is brought about. 



It seems unnecessary to pursue the ramifications of 

 the theory. It is a very pregnant one and has been of 

 inestimable value in enabling physiological chemists 

 to grasp the facts where, the true chemistry of the re- 

 actions being unknown, it might not have been possible 

 to follow them along conventional lines. 



But it will be remembered that our study of the 

 problems of immunity began with the resistance of 

 certain organisms to microparasites 

 by which others are successfully 

 invaded and destroyed, and though 

 these microorganisms were men- 

 tioned as the chief factors for con- 

 sideration, a digression was made 

 in order that the facts appertain- 

 ing to immunity from intoxication 

 might be thoroughly in mind, for 

 it is almost axiomatic that ability 

 to resist infection implies ability to 

 endure the toxic products of the 

 microparasite. 



We may therefore be justified in 

 supposing that when an immune 

 animal is found to destroy micro- 

 parasites in its body, it must be in- 

 different to their toxic products. 

 Its cells experiencing no injury are 

 and destroy the invading microorganisms. 



The destruction of the microparasites in immune ani- 

 mals is effected in two ways: 1, by the activity of the 

 phagocytic cells which devour them; 2, by solution in 

 the body juices. Both methods are usually observed, 

 though the former is most frequent in naturally immune 

 animals, the latter in animals with acquired immunity. 



Though perhaps not first observed by him, phagocy- 

 tosis was first suggested by Metchnikoff as the chief 



FIG. 137. Surface of a 

 cell with a receptor of the 

 second order fitted to com- 

 bine on one hand with a, 

 an albuminous molecule, 

 and 6, an enzymic mole- 

 cule. (Ehrlich and Mar- 

 shaU.) 



free to attack 



