248 Immnnity in Some of its Biochemical Aspects [Jan. 



Most germs, for some obscure reason, affect by preference cer- 

 taiii parts of the body. The typhoid bacillus usually lodges in the 

 wall of the small intestine; the meningococcus prefers the lining 

 membranes of the brain and spinal cord; the gonococcus is very 

 prone to attack the mucons membrane of the genital organs and of 

 the eye; the pneumococcus affects chiefly the respiratory organs; 

 the diphtheria bacillus lodges in the throat and nasal passages ; the 

 malaria parasite lodges only in the red blood cells; and certain 

 molds affect only the skin. 



Immunity. Natural immunity. It is very well known, 

 however, that certain infectious diseases occur naturally only among 

 some of the lower animals and do not affect man, while conversely, 

 others appear to attack only man. Among the latter may be men- 

 tioned typhoid fever, syphilis, gonorrhea. In speaking of the re- 

 sistance evidently possessed by certain species we make use of the 

 term natural immunity. Thus chickens and frogs possess a natural 

 immunity against tetanus (lockjaw) ; dogs, a natural immunity 

 against anthrax; goats, a natural immunity against tuberculosis ; 

 and man, a natural immunity against certain diseases of cattle. 

 This natural immunity, however, is not always absolute. Chickens, 

 for example, can be infected with tetanus if their bodies are chilled, 

 and frogs can be made susceptible to tetanus by keeping them un- 

 duly warm. 



Acquired immunity. Another form of immunity is that ob- 

 served in individuals who have had one attack of a particular in- 

 fection; thereafter they are practically safe from a second attack. 

 These individuals are said to possess an acquired immunity. This 

 form of immunity is well illustrated in scarlet fever, measles, small- 

 pox, yellow fever. Often this immunity lasts throughout the life- 

 time of the individual though there are exceptions. 



In studying this form of immunity, Pasteur conceived the idea 

 of artificially producing an attack of a given infection in order to 

 Protect the individual against another attack. He realized that it 

 was necessary, however, to so control matters that the original 

 attack should run a very mild course and not endanger the life of 

 the individual. After considerable experimental work, Pasteur 

 found that this could be accomplished by artificially weakening the 



