462 IMMUNITY 



afterwards more virulent, and by increasing the doses, a high 

 degree of immunity may be obtained. 



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. In 

 this method the so-called endotoxins will be injected along with 

 the other substances in the bacterial protoplasm, but the resulting 

 immunity is chiefly directed against the vital activity of the 

 organisms is antibacterial rather than antitoxic (vide infra). 

 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. This method was carried out with a great degree 

 of success in the case, of diphtheria, tetanus, malignant oedema, 

 etc. It appears capable of general application in the case of 

 organisms where it is possible to get an active toxin from the 

 filtered cultures. It has also been applied in the case of snake 

 poisons by Calmette and by Fraser, and a high degree of im- 

 munity has been produced. 



Immunity may also be obtained by means of certain chemical 

 substances separated from filtered bacterial cultures, though 

 these substances are generally in a more or less impure 

 condition. Hankin was the first to obtain this result by means 

 of an albumose separated from anthrax cultures. 



The following may be mentioned as some of the most 

 important examples of the practical application of the principles 

 of active immunity, i.e. of protective inoculation : (1) Inocula- 

 tion of sheep and oxen against anthrax (Pasteur) (p. 314); (2) 

 Jennerian vaccination against smallpox (p. 503) ; (3) Anti- 



