NATURAL AND ACQUIRED IMMUNITY 109 



immunity in man due to previous specific infection influences the 

 survival of many individuals during an epidemic. 



Antibodies and Natural Resistance. — Antibodies are specific 

 biochemical substances which may or may not be present in the 

 blood. They can be detected by the effect they have upon the 

 specific chemical su])stances (antigens) for which they have an 

 affinity. The antibodies, called antitoxins, neutralize their specific 

 toxins and under proper conditions the toxin-antitoxin mixture 

 may form a precipitate; the union of specific hemagglutinins to 

 homologous red cells in the presence of electrolytes results in the 

 agglutination oi- cliiiiiping of the cells. Likewise the union of 

 antibody with homologous bnctei'ia in the presence of electrolytes 

 results in the agglutination of tlie bacteria. 



When red cells or bacteria are united with their specific anti- 

 body, they become susceptible to dissolution or lysis by a normal 

 enzyme-like constituent of blood called complement. A more ex- 

 tensive discussion of antibodies is given in subsequent chapters. 

 Because of the specific relationship that exists between antibody 

 and its bacterial antigen, antibodies are regarded as specific im- 

 munity factors. Webster and Hodes (1939) conclude from their 

 studies of mouse typhoid that specific immunity factors play a 

 major role in infections with a liigh morbidity and a low mortality 

 rate and a minor role where the infection gains access to a herd 

 and s])reads for the first time or when the infection is associated 

 with a high mortality rate. 



Irwin and Hughes (1933) in their study of inheritance im- 

 munity in rats noted that death of infected animals seemed to be 

 correlated with absence of germicidal power in their blood. While 

 this suggests a possible correlation between immunity and anti- 

 body, yet many exceptions have been reported in the literature. 

 There is some evidence that the passive transfer of antibodies 

 from mother to offspring through mammary secretions or through 

 the placenta may confer temporary protection to the latter. This 

 is supported hj the folloAving research: Theobald Smith (1907) 

 demonstrated that in guinea pigs transient immunity can be trans- 

 mitted from mother to offspring through the milk. He has also 

 shown that the newborn calf acquires antibodies through the 

 colostrum from the mother. 



