ii8 ELEMENTS OF APPLIED MICROSCOPY. 



closely resembling the diphtheria organism, may be 

 present. 



7. The Serum Reaction in Typhoid Fever. — One of 



the most important practical results of the rscent 

 marvellous progress in the study of the phenomena 

 of immunity has been a method for the clinical diag- 

 nosis of typhoid fever and certain other diseases. It 



Fig. 46. — Diphtheria Bacilli (Methylene Blue). (After Wes- 

 brook.) Diagrammatic. 



has been shown that when the animal body is invaded 

 by a parasitic micro-organism, the cells produce in some 

 cases antitoxins which neutralize the poisons of the 

 microbe, and in other cases anti-bodies of another type 

 which have a specific destructive action on the parasite 

 itself. Sometimes the defensive secretions dissolve the 



