56 CLINICAL BACTERIOLOGY. 



found in blood-serum from cholera-cadavers, in a number 

 of cases, a protective influence that at times was wanting 

 in the blood of convalescents. We shall return to this 

 variation in the relations between existing immunity and 

 the immunizing power of the blood-serum. 



Quantitative Limitations of Immunity. Immunity is 

 always relative ; absolute immunity is theoretically incon- 

 ceivable. The susceptibility or the insusceptibility to dis- 

 ease, on the one hand, and the intensity of the disease, upon 

 the other hand, are more or less marked. All processes in 

 this connection stand in regular quantitative dependence 

 upon one another. A mathematically accurate measure of 

 these variations in degree is at present impossible, as the 

 bacterial poisons have thus far eluded chemic analysis, and 

 a unit of measure from which to start out, therefore, does 

 not yet exist. Approximately correct investigations have, 

 however, been made more particularly in the case of 

 tetanus and of diphtheria, whose bacterial cultures contain 

 powerful poisons that pass over into the germ-free filtrate 

 of bouillon-cultures. It has been shown in this connection 

 that a definite amount of the immunizing culture establishes 

 a definite degree of immunity, that with further introduction 

 of the immunizing substances the degree of immunity also 

 increases, and finally that the immunizing power of the 

 blood-serum increases to a certain degree with the degree 

 of the existing immunity. It is obvious that the pro- 

 tection conferred by inoculation is never absolute. A de- 

 gree of immunity, however high, can protect only against 

 a definite degree of disease ; if the highly immune animal is 

 infected with an amount of bacteria or of toxin representing 

 a still higher degree of disease, it will finally succumb. 

 The highest degree of immunity that is known is probably 

 that possessed by fowl to tetanus, which sufficiently pro- 

 tects against all danger of infection that exists in nature ; 

 there is no natural tetanus among fowl. Even this high 

 degree of immunity may, however, be unable to withstand 

 an excessive degree of intoxication in laboratory -experimen- 

 tation, and, in fact, tetanus has thus been developed in a 

 typical manner in fowl and pigeons. 



Specificity of the Immunity. Immunity is, in general, 

 specifically limited. The immunity conferred by vaccina- 

 tion protects only against smallpox ; that which follows 

 recovery from an attack of scarlet fever does not protect 



