IMMUNOLOGY. IMMUNITY AND IMMUNIZING AGENTS 253 



goat. Blood immunized against the ape will form precipitate with 

 human blood. The test has been used practidally in medico-legal and in 

 criminal cases. The details for making the test may be found in the 

 larger works on bacteriology, parasitology and on immunization. The 

 method is given in full in Bacteriological Methods in Food and Drug 

 Laboratories. P. Blakiston's Son and Company. 1915. (.Schneider.) 



h. Virulency — Aggressins. — The pathogenic bacteria form substances 

 which protect against the defensive measure of the host, however, one 

 and the same species of pathogenic or toxigenic organism is not at all times 

 equally active defensively. In other words, the virulency of bacteria is 

 variable. In the manufacture of diphtheria antitoxin it is desirable to 

 use a strain of the causative organism which is highly virulent, in order to 

 hasten as well as to increase the formation within the blood of the horse, 

 of the defensive or protective bodies. ^ The virulency may be lowered or 

 weakened (attenuated) in various ways. Exposure to high temperatures 

 may accomplish this. In fact the high bodily temperature in fevers is 

 nature's method of reducing the virulency of the invading organism. 

 Again, the virulency may be lowered and completely modified qualitatively 

 by a change of host, as in cow pox. An organism maybe entirely harmless 

 and even highly useful in one position, and become highly virulent in 

 another position in the same organism. Thus the B. coli is a normal and 

 beneficent inhabitant of the intestinal tract, but should it be introduced 

 (accidentally or by design) into the peritoneal cavity or into any tissue 

 other than the intestinal tract, it may set up serious abscess formation. 

 The virulency of the causative agent of rabies is lowered as well as modi- 

 fied by exposure to dry air. In a general way all agencies which lower the 

 vitality of pathogenic and toxigenic organisms tend to lower the virulency, 

 although there are some notable exceptions. 



As it is possible to lower (attenuate) as well as to mcrease (augment) 

 the activity (virulency) of objectionable microorganisms, just so is it 

 possible to increase the activity of useful organisms. Thus the free nitro- 

 gen assimilatmg power of the Rhizobia group may be greatly increased 

 by growing the organisms upon special media, or we may reduce this 

 power greatly or practically destroy it. To such changes in beneficent 

 organisms we apply the term potency, rather than virulency. We in- 

 crease or augment the potency of a yeast ferment, and on the other hand 

 we increase or augment the virulency of the diphtheria germ. 



The subject of increased virulency of pathogenic organisms has re- 

 ceived a great deal of attention within recent years. Bail and others 

 made some interesting observations which may be summarized as follows: 

 By injecting pure cultures of tubercle bacUli into the peritoneal cavity 

 of a guinea pig, a rapidly fatal tuberculosis is produced. If the peritoneal 



