260 president's address — section l, 



No' doubt these differences in susceptibility, or, in other words, 

 different degreej of natural immunity ainong diiferent species of 

 animals to the same micro-crganism, are related to characteristic 

 specific chemical or physical differences in the blood and cytoplasm 

 of the different animals. These differences are, however, much too 

 refined and subtle to be put in evidence by ordinary chemical 

 means, but the specific actions t)i vitro of "precipitins." and 

 '' hgemolysins " on homologous sera and cells afford a s.trLking 

 denronstration. Thus therei is an intimate relation between the 

 special powers of a particular organism to infect and the special 

 defence measures of the particular animal species used for self- 

 protection. 



Apart from such subtle specific constitutional differences of sus-- 

 ceptibility, there is another line of defence which appears to be 

 efficient just in proportion as the particular animal is immune 

 or insusceptible, namely phagocytosis. Where, in a particular 

 species of animal, phagocytosis of a particular micro-organism 

 is vigorous and complete, there is no infection. This can 

 be readily observed after experimentally inoculating an 

 animal with an organism having little virulence for that 

 particular species. That the blood plasma and body fluids also 

 help in the defence of the body of a normal animal is not in 

 doubt, but the chief agents in keeping the body free from invaders 

 are the phagocytes. By all the tests which can be applied toi deter- 

 mine the strength or titre of the serum immunity in the case of 

 acquired immunity (agglutinins, opsonins, bacteriolysins, comple- 

 ment fixation), the immunizing effect of the serum, as distinct from 

 the ceils, in natural im.munity is small. The effects of the blood 

 serum are much more evident in cases of acquired immunity, and 

 can be studied better in that condition. 



A cquired Im.munity . 

 Acquired immunity is a relatively marked insusceptibility to 

 infection with a particular species of micro-organism, produced as 

 a result either of a previous attack of thei same disease, or <if 

 vaccination. Under both these conditions, it appears to be the 

 introduction of an antigen, or group of antigens, which has given 

 rise toi antihody formation. Long before the etiology of disease 

 was understood, the immunity resulting from a first attack of 

 many diseases was well accredited. Experimental introduction of 

 antigens brings about thei same^ insusceptibility, and careful 

 analysis of the phenomena has led at least to a partial understand- 

 ing of the mechanism. 



It will be suff cient for the moment to^ mention the main i-esults 

 of acquired immunity in the prod action of defences additional to 

 those seen in cases of natural immunity. The accumulation of " anti- 

 toxins " in the body fluids of an animal inoculated secundum artevi 

 Mith a "toxin"; the presence of "agglutinins" in the blood 



