348 IMMUNITY AND CURATIVE INOCULATIONS 



" immunity " to the anthrax disease conferred on cattle 

 and sheep by Pasteur's inoculations of weakened, but still 

 actively growing, cultures of the anthrax bacillus. 

 Another theory was that they produced something in 

 the blood by their own life-processes which checked their 

 further growth, just as yeast will not grow in wort in 

 which it has produced 8 per cent, of alcohol, and as a fire 

 may be choked by its own smoke or ashes. 



We now know that both these explanations of " im- 

 munity" are incorrect. Nature provides at least three 

 varieties of defence within the blood of higher animals 

 against disease-producing microbes which have broken 

 through the outer line of fortification, the skin. These 

 three methods are effective in different cases (one in this 

 disease, the other in that), and, on the whole, are sufficient 

 to preserve the races of animals (including man) from 

 complete destruction. These are (i) the production in 

 the blood of an antidote to the toxin or poison elaborated 

 by the invading microbe — an antitoxin, which chemically 

 neutralises the toxin ; (2) the production in the blood of 

 the attacked animal of a " germicidal " poison which 

 repels and kills the attacking microbes themselves (not 

 merely neutralising their poisonous products) ; (3) the ex- 

 termination of the intrusive, disease-producing microbes 

 by a kind of police, which scour th6 blood channels and 

 tissues and " eat up " — actually engulf and digest — the 

 hostile intruders. These latter agents, actual particles of 

 the living animal in which they exist, are the " eater- 

 cells," or " phagocytes " — minute, viscid, actively moving 

 cells, resembling the animalcules called " amoeba." They 

 are only the one two-thousandth of an inch in diameter, 

 and are known as the white or colourless corpuscles 

 of the blood. They are far less numerous than the 

 red blood-corpuscles, which are the agents for carrying 

 oxygen, but there are eight thousand million of them in 



