Immunity 197 



still again, various poisonous substances may be formed by the parasites 

 themselves, which will injure the immediately surrounding tissues of 

 the host only in the location of the parasites ; or, the poisons may be 

 soluble in the blood stream and in this way pass throughout the entire 

 body, injuring many regions. And there is still another way by which 

 injury is brought about by parasitical invaders. There are certain 

 bacteria and protozoa which require considerable oxygen for their life 

 processes. The red blood corpuscles have become red by coming in 

 contact with the air in the lungs and absorbing oxygen which they then 

 distribute throughout the body. If the parasite, however, takes this 

 oxygen from the red blood cells, only carbon dioxide and, under certain 

 conditions, carbon monoxide remains. Carbon monoxide is a constitu- 

 ent of "coal gas," which often asphyxiates men working in coal mines. 

 It, therefore, follows that one actually may be "gassed" and die of this 

 "gassing," if the body contains parasites which remove the much-needed 

 oxygen from the red blood corpuscles. In such instances where oxygen 

 is withdrawn, death results almost immediately. 



Then there may be all manner of mixed infections. Just as it 

 requires fire in order to cause powder to explode, so there are certain 

 chemical substances, as well as living organisms, which by themselves 

 do little harm or injury; but, when a second or third substance mixes 

 with them, may prove quite injurious. Conversely, a single substance 

 may be quite injurious, such as either an acid or an alkali ; but, when 

 the two are mixed, they neutralize each other, and no active injury is 

 brought about. 



Everyone knows that no two people are exactly alike in their ability 

 to resist disease, and that one person may tolerate a much greater injury 

 than another without succumbing to it. Most of us have probably read 

 of the ancient king who, being afraid that an enemy might poison him, 

 took small doses of various poisons daily so that in due time he could 

 take great quantities without its having any injurious effect upon him. 

 That is, his toleration for this specific poison grew, and his body was 

 able to resist the usual injury caused by such poison. That is, his system 

 became insensible to specific poisons which were thus unable to affect 

 him injuriously because an immunity to these poisons had been set up. 



Resistance, tolerance, and immunity may be classified in various 

 ways, such as racial, familial, and individual. As an example of race 

 immunity we have those groups of individuals living in the tropics 

 who do not succumb to the various tropical fevers that affect a stranger 

 almost immediately. The classic example, however, is that of the Jews, 

 who having fought tuberculosis for thousands of years, are now more 

 immune than any other known race of mankind. The Negroes and 

 Indians, on the contrary, never having had tuberculosis until the white 

 man brought it to them, succumb quickly. 



If certain families seem to be more or less immune, we call this a 



