Immunity acquired by natural means 437 



investigators. The reaction of the tuberculous organism against re- 

 infection has received the name of " Koch's phenomenon." 



Clinical medicine has afforded many data of the highest import- 

 ance bearing on the establishment of an acquired immunity in many 

 infective diseases ; but a scientific study of the mechanism of this 

 immunity could only be founded on the result of microbiological 

 researches obtained during the recent period of scientific activity. 

 The general conclusion to be drawn from these researches is that the 

 immunity, acquired by natural means, is very analogous to that which 

 is obtained artificially by vaccination by the various methods already 

 mentioned. The phenomena observed in animals inoculated with the 

 various known vaccines present a great resemblance to those that 

 obtain during recovery from a disease contracted under natural con- 

 ditions. To support this thesis it would be necessary for us to survey 

 the mechanism of healing, which would carry us too far afield, the 

 subject being far too vast to be summarised here. We must, then, 

 content ourselves with a few remarks inserted for the instruction 

 of the reader on this subject. 



Those diseases against which no remedy exists are most suitable 

 for furnishing us with important information on immunity acquired 

 by natural means. We have already seen in the case of malaria to 

 what point therapeutic treatment can modify the natural course of 

 the phenomena. For this reason it will be useful to consider first the 

 immunity acquired as the result of a first attack of typhoid fever. 

 The immunity which develops in this example is both marked and 

 persistent ; the therapeutic intervention which might disturb the 

 natural phenomena is nil. 



As yet we do not know the mechanism of healing in typhoid fever. 

 This disease affecting the human species exclusively (the experi- 

 mental peritonitis of animals, set up by the typhoid coccobacillus, is 

 distinguished by very marked differences), it is very difficult to find a 

 means of studying it at all satisfactorily during the phase of recovery. 

 Even in default of this knowledge, however, it is possible to gather [459] 

 some idea as to the changes which the blood plasma undergoes, not 

 only during the course of an attack of typhoid fever, but also during 

 and after convalescence. 



Some time ago Chantemesse and Widal 1 observed that the blood 

 serum of persons attacked by typhoid fever acquires the property of 

 inhibiting the experimental peritonitis set up by the typhoid cocco- 

 1 Ann. de Flnst. Paslcur, Paris, 1892, t. vi, p. 773. 



