PROTEINS 499 



significant substances (page 492). This does not alter the fact 

 that the proteins are probably specific — specific for the sub- 

 stances with which they are associated, specific for a given 

 species, and specific for individual differences in a species. The 

 following are other better authenticated examples of protein 

 individuality — a preferable expression for what we here have in 

 mind, because while both individuality and specificity are based 

 on stereochemical congruity, protein specificity implies, in 

 immunology, an elective chemical affinity {e.g., the reaction 

 between antigen and antibody). 



One individual's susceptibility and another's immunity to 

 poisons, bacteria, proteins, and toxms in general are problems in 

 protein individuality, as is also the fact that the sperm of only 

 the same species (and not even always then) can fertilize an 

 egg; that only certain animals can save themselves from certain 

 diseases through the production of an antitoxin which counter- 

 acts the toxin {e.g., the horse in protecting itself against diph- 

 theria) ; that the geranium is susceptible to the plant-gall bacillus, 

 and many other plants not; that the chestnut is susceptible to a 

 blight caused by the fungus Endothea parasitica, while other 

 trees are not; that man is immune to the distemper of dogs and 

 highly resistant to the foot-and-mouth disease of cattle; that 

 the hair and feathers of most domestic animals are poisonous to 

 some persons but not to others ; that nearly all people are sensitive 

 to one or more of 150 common foods among which strawberries 

 and eggs are the chief offenders. 



Substances other than proteins may at times be the cause of an 

 immunity or sensitivity; consequently, chemical rather than 

 protein individuality is the more exact expression. Fats and 

 carbohydrates occasionally appear to be responsible {e.g., in the 

 case of the capsular carbohydrate of pneumococci). 



Immunology embraces the problems that we have been 

 considering. It is the study of the resistance of the living 

 organism to pathogenic or disease-producing agents. In addi- 

 tion to a natural or inherited immunity, there is an acquired 

 immunity. We become immune to measles after the first 

 attack, but we do not inherit the acquired immunity that our 

 parents developed. Acquired immunity can be obtained in 

 three ways — by getting a mild case of the disease ourselves, by 

 vaccination, or by taking advantage of the immunity acquired 



