58 INFECTION AND RESISTANCE 



we must remember that they were carried out with antitoxic im- 

 munity only, in which the resistance is purely dependent upon the 

 circulating antibody and is never, even in actively immunized indi- 

 viduals, a permanent state. In immunity such as that acquired 

 against typhoid fever, plague, cholera, and other diseases after re- 

 covery from an attack, the individual remains relatively resistant 

 long after the demonstrable antibodies have disappeared from 

 the circulation, and we must assume that this permanent re- 

 sistance depends upon a physiological alteration inexplicable for 

 the present, but surely residing in the body cells. In such cases 

 it is by no means certain that there may not be a very slight, but 

 through generations gradually accumulating, inheritance of im- 

 munity. At any rate the experiments of Ehrlich do not disprove 

 such a possibility. Moreover, in this connection it must not 

 be forgotten that natural immunity, unlike acquired immunity, 

 cannot be passively transferred from one animal to another, and 

 implies therefore a fundamental cellular difference rather than 

 a condition depending merely upon antibodies circulating in the 

 blood. 



For this last reason also it has been unsatisfactory to attempt 

 explanations of natural immunity purely upon grounds of bacteri- 

 cidal and other properties of the blood serum. These points we will 

 take up at greater length when we discuss the mechanism of resist- 

 ance in general. 



An important observation upon the inheritance of serum prop- 

 erties is that which has been made by Ottenberg and Epstein n in 

 connection with the iso-agglutinins. We shall see in another section 

 that the blood serum of one human being will often possess the 

 property of agglutinating the human blood cells of another indi- 

 vidual. These iso-agglutinating constituents of the serum are ap- 

 parently transmitted from parents to offspring. Von Dungern and 

 Hirschfeld, 12 in studying these iso-agglutinins in 72 families, upon 

 348 people, not only confirmed the observations of the preceding 

 workers, but showed that such inheritance follows Mendelian laws. 

 Not only is this of great biological interest, but it is of great im- 

 portance in connection with our present discussion in showing that 

 such properties as agglutinating powers of serum can be influenced 

 by inheritance from the father as well as from the mother. 



The individual differences in resistance which unquestionably 

 exist among members of the same species and races are very difficult 

 to explain, but, as far as we can tell anything about them at all, they 

 seem to depend upon variation in what is popularly spoken of as 

 "general condition." The laboratory animals with which most ex- 

 perimentation is done, rabbits and guinea pigs, if healthy, show very 



11 Ottenberg and Epstein. Proceedings of the N. T. Path. Soc., 1908. 



12 Von Dungern and Hirschfeld. Zeitschr. f. Immunitats., Vol. 4, 1910. 



