BY BACTERIAL PRODUCTS OR TOXINS. 467 



oedema by being injected with the bacillus prodigiosus ; an 

 attenuated streptococcus by being injected with the bacillus coli, 

 etc. A culture of the typhoid bacillus may be increased in 

 virulence, as already stated, by being injected along with a dead 

 culture of the bacillus coli. In such cases the accompanying 

 injection enables the attenuated organism to gain a foothold in 

 the tissues, and it may be stated as a general rule that the 

 virulence of an organism for a particular animal is raised by its 

 growing in the tissues of that animal. 



Combination of Methods. The above methods may be com- 

 bined in various ways. By repeated injections of cultures at first 

 attenuated and afterwards more virulent, and by increasing the 

 doses, a high degree of immunity may be obtained. This is 

 well exemplified in the case of Haffkine's method of anti-cholera 

 inoculation (p. 421). 



2. Immunity by Dead Cultures of Bacteria. In some cases 

 a high degree of immunity against infection by a given microbe 

 may be developed by repeated and gradually increasing doses 

 of the dead cultures, the cultures being killed sometimes by 

 heat, sometimes by exposure to the vapour of chloroform. Some 

 consider that in this method only the intracellular toxic sub- 

 stances of the organism are introduced when the cultures have 

 been taken from the surface of a solid medium, such as agar, but 

 as the surface is moist, some of the extracellular products must 

 be present also. The cultures when dead produce, of course, less 

 effect than when living, and this method may be conveniently 

 used in the initial stages of active immunisation, to be afterwards 

 followed by injections of the living cultures. The method is 

 extensively used for experimental purposes, and is that adopted 

 in anti-plague and anti-typhoid inoculations. 



3. Immunity by the Separated Bacterial Products or Toxins. 

 The organisms in a virulent condition are grown in a fluid 

 medium for a certain time, and the fluid is then filtered through 

 a Chamberland or other porcelain filter. The filtrate contains 

 the toxins, and it may be used unaltered, or may be reduced in 

 bulk by evaporation, or may be evaporated to dryness. The 

 process of immunisation by the toxin is started by small, non- 

 lethal doses of the strong toxin, or by larger doses of toxin the 

 power of which has been weakened by various methods (vide 

 infra). Afterwards the doses are gradually increased. Im- 



