558 IMMUNITY 



which was originally' introduced' by Besredka, the bacterial 

 emulsion is treated with the corresponding anti-serum, that is, 

 the serum of an animal immunised- against the particular 

 bacterium, and after being left in contact for some time, the 

 serum is separated by the centrifuge and the bacteria are 

 thoroughly washed free of all traces of serum. The bacteria 

 thus treated constitute the vaccins sensibilizes. It is claimed 

 that, while immunity produced by them is rapidly developed 

 anjl is of long duration, the local toxic effects on subcutaneous 

 injection are very much lessened. The method has been applied 

 in vaccination against typhoid, plague, cholera, and dysentery. 

 Apparently in such sensitised vaccines the antigen molecules of 

 the bacteria will be largely combined with anti-substances, and 

 thus on theoretical grounds we would expect that only those 

 molecules left free, or those which become free by dissociation, 

 will be able to act as antigens and the antigenic power of the 

 bacteria will be diminished. Certain observations show that this 

 is the case, but it would be desirable to have fuller knowledge of 

 the amounts of anti-substances developed by the sensitised and 

 non-sensitised bacteria respectively and of the relation of such 

 amounts to the degree of protection afforded. 



4. 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 and tetanus. 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 venoms by Calmette and by Fraser, 

 and a high degree of immunity has been produced. 



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. 347) ; (2) 

 Jennerian vaccination against smallpox (p. 609) ; (3) Anti- 

 cholera inoculation (Haffkine) (p. 473) ; (4) Anti-plague 

 inoculation (Haffkine) (p. 498) ; (5) Anti-typhoid inoculation 



