20 THE PRINCIPLES OF IMMUNOLOGY 



Family Immunity. Members of certain families may through 

 generations appear to be especially susceptible to such diseases as 

 tuberculosis and rheumatism or the converse may be true. In the 

 case of tuberculosis this difference may be the result of conforma- 

 tion of the body. The physical character of flat, narrow chest and 

 thin skin apparently go hand in hand with susceptibility to tuber- 

 culosis, whereas the well-rounded chest appears to indicate resist- 

 ance. In a family with whose history we are familiar the blondes 

 have almost invariably succumbed to tuberculosis and the brunettes 

 living under the same conditions and in intimate association have been 

 resistant. This must be due to inherent constitutional characters and is 

 not to be considered as a difference due to complexion alone. 



Individual Immunity. Variations of individual resistance or im- 

 munity are seen frequently. It is true that the extremes of age 

 show a certain proneness to infection and that this varies somewhat 

 with individuals. Excellent examples of individual resistance are 

 seen in great epidemics where some of those exposed apparently in 

 the same manner and under the same hygienic conditions as others 

 show either complete resistance to the disease, or, if they are at- 

 tacked, develop only moderate or slight attacks. Infected water and 

 foods consumed by a population may lead to disease in only a small 

 portion of those exposed. Individual variations in animals are very 

 frequent and offer a considerable source of error in the interpreta- 

 tion of experimental results. If a series of guinea-pigs be injected 

 with the same dose of anthrax bacilli, all will die at practically the 

 same time, but if rabbits be treated in the same way some die 

 within two days, others die subsequently, and still others are com- 

 pletely resistant. On the other hand, rabbits are all susceptible to 

 chicken-cholera, whereas the guinea-pig shows great individual dif- 

 ference. Although a large number of children suffer from tonsillar 

 infections, yet the incidence of acute articular rheumatism or of 

 endocarditis is small and variable. Instances might be multiplied 

 indefinitely of individual variations in resistance, but the phenom- 

 enon is one of common knowledge. 



Inherited Immunity. The immunity transferred from parent to 

 offspring may be a natural immunity or an immunity acquired by 

 the parent. The transfer of natural immunity may be seen in racial, 

 species, and family manifestations, and is probably a true transfer 

 through the germ plasm. Congenital immunity may arise either in 

 the form of an active immunity developed in the fetus because of 

 the presence of antigens in the circulating blood of the mother, or 

 may be in the form of passive immunity transferred from the blood 

 of the mother to that of the fetus. It is conceivable that the fetus 

 may survive an attack of disease transmitted from the mother and 

 thereby become immune. It has been known since the time of 

 Pasteur that certain dogs are immune to rabies. Remlinger has 

 found that the guinea-pig may transfer rabies to the fetus and 

 puppies have been known to become rabid several months after 



