XI 



Antibodies and Immunity 



Without special means of protection large organisms could not live 

 very long in a world full of parasitic bacteria and viruses. It is diffi- 

 cult to think of a more perfect breeding ground for them than animal 

 tissues and fluids. One method of protection developed by bacteria 

 themselves and by fungi is the use of antibiotics — substances which 

 prevent the growth of foreign bacteria and so secure for their pos- 

 sessor a certain amount of living space. It is possible that animals and 

 plants have on their outer surfaces antibiotic substances which in- 

 hibit fungal growths — but such have not been clearly demonstrated. 



The skin or outer layer of cells is hard and impervious which helps 

 to keep parasitic particles out of the organism, but it is not sufficient 

 to prevent the accidental entry of micro-organisms through scratches 

 and cuts and through the softer tissues of the alimentary canal. For 

 this reason, it has been necessary for animals to develop other means 

 of protection against the invasion of the body by foreign organisms. 



The facts which imply the existence of internal protective agents 

 have been known from early times. It is common knowledge that 

 when an individual has recovered from an infectious disease like 

 measles or mumps, he is immune from a second attack even if he re- 

 ceives another infection. It was also found that with certain diseases 

 which occur in two forms, one mild and the other virulent, infection 

 with the mild form often confers immunity against the virulent form. 

 This was the case with cowpox and smallpox and in some countries 

 it has long been a practice to seek to be infected with cowpox for the 

 sake of immunity against the more serious disease. In England vac- 

 cination with cowpox to confer immunity against smallpox was in- 

 troduced successfully by Edward Jenner. The same line of thought 

 led to the attempt to produce mild strains of a disease for the specific 

 purpose of conferring immunity. 



Experiments with laboratory animals showed that the injection of 



