130 



INFECTION AND IMMUNITY. 



Family Sus- 

 ceptibility 

 and 

 Immunity. 



fected with scarlet fever or measles, nor man with 

 chicken cholera. The negro is less susceptible 

 than the white man to yellow fever. The resist- 

 ance which these examples illustrate exists natur- 

 ally, not through having the disease ; it is a natural 

 immunity. 



Natural immunity is, for the most part, an in- 

 herited condition; this certainly is the case where 

 a whole class of animals is involved. Similarly, 

 the susceptibility which is peculiar to a species 

 must be hereditary. It is often said of some dis- 

 eases that they "run in families " e. g., carcinoma, 

 gout, insanity. This appears to be just as true of 

 some infectious diseases, the most noteworthy ex- 

 ample of which is probably tuberculosis. In con- 

 trast to this inherited susceptibility is an inherited 

 immunity, which may also "run in families/' It is 

 not so easy to adduce examples of this. We are in 

 the habit of thinking of the individual who can 

 resist all infections as representing a standard. 

 He, however, is above the average in resistance, 

 and the average is our proper standard for esti- 

 mating the resistance of a species or race of ani- 

 mals. It is undoubtedly true that some families 

 possess an unusual resistance to tuberculosis. 

 Furthermore, experimental work with animals has 

 proved that, within limits, an immunity to cer- 

 tain infections (e. g., tetanus) acquired by a fe- 

 male may be transmitted to her offspring. Such 

 immunity, however, is very transient in character, 

 and is "passive" in its type; it depends on the 

 transfer of protective substances from the circula- 

 tion of the female parent to that of the embryo 

 in utero, and some weeks after the birth of the lat- 

 ter these substances are eliminated and the immun- 



