138 THE KINGDOM OF MAN 



eater-cells, and in his beautiful book on Inflammation, 

 published twenty years ago, proved the extreme impor- 

 tance of their activity. At the same time he had shown 

 that they eat up intrusive bacteria and other germs (see 

 figs. 37 to 43) ; and his work for the last twenty years has 

 mainly consisted in demonstrating that they are the chief, 

 and probably the only, agents at work in either ridding 

 the human body of an attack of disease-causing germs or 

 in warding off even the commencement of an attack, so 

 that the man or animal in which they are fully efficient 

 is ' immune' that is to say, cannot be effectively attacked 

 by disease-germs. 



Disease-germs, bacteria, or protozoa produce poisons 

 which sometimes are too much for the phagocytes, poison- 

 ing them and so getting the upper hand. But, as Metsch- 

 nikoff showed, the training of the phagocytes by weak 

 doses of the poison of the disease-germ, or by weakened 

 cultures of the disease-germ itself, brings about a power 

 of resistance in the phagocytes to the germ's poison, and 

 thus makes them capable of attacking the germs and 

 keeping them at bay. Hence the value of inoculations. 



The discussion and experiments arising from Metschni- 

 koif s demonstrations have led to the discovery of the 

 production by the phagocytes of certain exudations from 

 their substance which have a most important effect in 

 weakening the resistance of the intrusive bacteria and 

 rendering them easy prey for the phagocyte. These are 

 called ' sensitisers,' and have been largely studied. They 

 may be introduced artificially into the blood and tissues so 

 as to facilitate the work of the phagocytes, and no doubt 

 it is a valuable remedial measure to make use of such 

 sensitisers as a treatment. Dr. Wright considers that 

 such sensitisers are formed in the blood and tissues inde- 

 pendently of the phagocytes, and has called them ' opso- 



