INFECTION AND IMMUNITY. 
35 
tion of the poisons or toxins made by the bacteria. 
These poisons may be extracellular or intracellular. 
The extracellular toxins are thrown out of the bodies 
of the bacteria into the tissues or media in which they 
are growing. The word toxin when used alone is 
taken to mean an extracellular toxin. The intracel¬ 
lular or endotoxins are retained within the bodies of 
the bacteria and are set free only after their death or 
dissolution. After absorption the bacterial toxins do 
not affect all organs or tissues equally, but exhibit a 
selective action, some attacking the red blood-cor¬ 
puscles and dissolving them, others the tissues of the 
brain and nervous system. 
One might think, from what has been said, that 
men and animals are wholly at the mercy of bacteria. 
Fortunately this is not so, as all are endowed with 
certain defensive powers that resist the injurious 
action of bacteria and their poisons. This resistance 
to disease is called immunity. 
Many of the diseases that are infectious in man 
cannot be transmitted to animals and, conversely, some 
of the infectious diseases of animals do not occur in 
man. 
Among the races of men variations in the resist¬ 
ance to disease is observed; for example, the negro 
seems to possess a much greater resistance to infection 
with yellow fever than the white man. In addition to 
the variations in resistance among the races of man 
there are also variations among individuals. The 
conditions under which people live have much to do 
Toxins, 
extra- and 
intra¬ 
cellular 
Immunity 
Natural 
immunity 
