62 
PRINCIPLES OF BACTERIOLOGY 
Just how this is brought about we do not know, but 
the fact remains that in studying the immune bodies we 
always speak of them as occurring in the blood serum 
although in the living body there is no such thing as 
blood serum (the latter existing only in the blood out¬ 
side of the body). 
Now after this short digression, let us return to the 
consideration of the immune bodies or antibodies. 
The first statement to make in this connection is that 
the normal blood has the power to destroy bacteria; 
this power varies in different individuals; if the blood 
serum is kept for some time it will be noticed that this 
power rapidly diminishes. The substance responsible 
for this effect of blood is called complement. 
We now know that there are two chief kinds of bac¬ 
terial poisons—the exotoxins and the endotoxins, the 
former being the separable poisons, secreted by bac¬ 
teria and circulating in the blood throughout the en¬ 
tire body, while the bacteria themselves do not enter 
the blood circulation and usually remain where they 
have become lodged; the latter are inseparable poisons 
and only are liberated from the bacterial body after 
the bacteria have died, the bacteria themselves having 
circulated in the blood (the third kind of bacterial 
poisons, namely, the bacterial proteins are unimportant 
since they are not specific). It is evident that since the 
poisons liberated by the bacteria are different, the anti¬ 
bodies must be different, and such is the fact. 
Let us first consider the case of the exotoxin-producing 
bacteria, such as the diphtheria or tetanus bacilli; when 
these have lodged in the body, within a short time there 
will be produced an enormous amount of exotoxin; if 
the body should produce such substances that would 
kill the bacteria, this would be of very little use, be¬ 
cause while it would prevent further elaboration of 
