576 MICRO-ORGANISMS AND DISEASE [chap. 



toxic and immunising power by subjecting the horse to 

 repeated injections with large doses of living diphtheria 

 bacilli ; in one horse already after three weeks, in another 

 after four weeks, the serum had the same immunising power 

 as in horses done after Roux's method with pure toxin for a 

 considerably longer period. 



The immunising power of antitoxic diphtheria serum, 

 i.e. the power of the serum when injected into a normal 

 guinea-pig to protect the animal against subsequent in- 

 fection with the diphtheria bacilli, is only of a ten\porary 

 character, being of short duration, generally from a few 

 days to a few weeks, and dep>ends on the amount of serum 

 injected. The immunising action of the injection into a 

 guinea- pig of a subfatal dose of living culture of bacillus 

 diphtheriae is of considerably longer duration, but it must 

 be added that in that of the bacillus of diphtheria, un- 

 like with some other microbes — anthrax, cholera, typhoid, 

 colon, &c. — the resistance of the guinea-pig against new 

 and further infection is comparatively limited. 



It is clear from the facts above recorded that during the 

 process of " active immunisation" as first practised by 

 Behring and Kitasato, Behring, Roux, and others, the blood 

 (and blood-serum) of the immunised animal contains sub- 

 stances, atititoxins or antibodies, as a result of the antecedent 

 injections of toxin. Of what nature are these bodies ? Are 

 they a result of the activity of the cells and tissues, a re- 

 active secretion of new substances (ferment) by the cells 

 in consequence of successful and effective stimulation by 

 the toxin (Roux), or are they the original toxin modified 

 and chemically altered by the tissue cells (Buchner) ? Are 

 they of the nature of albumins like the toxalbumins, or are 

 they bodies more resembling ferments ? 



As regards the diphtheria antitoxins Aronson's mode of 



