PRESIDENT S ADDRESS — SECTION L. 259 



Time forbids more than a glimpse at much of the development of 

 the science of immunity, and one can only in passing pay tribute to 

 Jenner's magnificent anticipation in publishing before the Royal 

 Society of London, in 1796, his method of vaccination against 

 small-pox in man. 



The foundation for scientific prophylaxis was really laid by 

 Pasteur in 1867, when he demonstrated a " contagium vivum " 

 in an epidemic disease of silkworms. 



The masterly work of Metchnikoff en phagoicytosis of bacteria, 

 first observed m 1883, led to an appreciation of the importance of 

 one oi thei first aad most important lines of defence. Shortly 

 afterw^ards, Nuttall (1888) deincnstrated the bactericidal effect 

 of normal blood serum and other body fluids. In the same year 

 Richet and Hericourt made the first serious attempt to bring about 

 immunization of an animal by the injection of defibrinated blood 

 from a vaccinated, and, therefore, immune animal. Thus began the 

 controversy between the two schooils of thought, the upholders ot 

 the "cellular" theory of immunity led by Metchnikoff, and the 

 supporters of the " humoural " theory, led at first by Nuttall and 

 Buchner, and then dominated by the genius of Ehrlich. The last 

 thirty years has witnessed the gradual rapjrrochenwnt and mutual 

 acceptance with modifications of the two factors in defence. 



In our short survey of the present understanding of immunity, 

 it is convenient to take certain phases, and deal with them in 

 turn . 



First it will be well to glance at the fundamental facts of natural 

 immunity, and then pass to consider the j3i'ocesses by which 

 acquired immunity is brought about. 



Natural I mm uniti/. 



One of the most remarkable, features of disease infection in 

 animals is the susceptibility oi one species and the insusceptibility 

 of another to the same disease. Moreover, the species highly resis- 

 tant to one disease may be extremely susceptible to another. On 

 the other hand the characters of virulence and pathogenicity on 

 the part of bacteria are relative only, and these ternis can only 

 be used when applied to the action of a definite bacterium on cer- 

 tain animals susceptible to infection by it. A familiar example of 

 this selective action is provided by the various strains of the 

 tubercle bacillus. Cattle, so commonly affecteid with the bovine 

 strain, against which their resistance is of a. fairly low order, are 

 highlv resistant to infection with the human bacillus, and are quite 

 immune to the avian type. Again, the guinea pig is easily 

 infected with a progressive fatal infectioai by either human or 

 bovine strains, whereas it usually successfully resists infectioai with 

 the avian type. The rabbit, on the other hand, whilst being very 

 susceptible to the bovine type, is refractory to the human type. 



