74 
PRINCIPLES OF BACTERIOLOGY 
stance, present in all normal as well as immune blood 
serum, and which is destroyed by heating up to 56° 
C.; this normal substance is the same substance which 
has been mentioned before in the section on immunity 
and to which a slight normal bactericidal (bacteria- 
ldlling) property of the blood is due; it is called com¬ 
plement. 
We thus see that the receptors of the third order, 
such as bacteriolysins or hemolysins (which destroy the 
red blood cells), do not act upon bacteria or red blood 
Fig. 14.—The receptor of third order (bacteriolysis, hemolysin). The 
receptor (side-chain or immune body), b consists of two parts; c, which 
unites with bacterium e and d which unites with complement (/). 
cells, as the case may be, alone, but in conjunction with 
the complement; because of this the side chain receptor 
of the third order has two “chains,” one by which it 
combines with the bacterium oP red blood cell or what¬ 
ever had been injected into the animal, and the other 
for combining with the complement. (Fig. 14.) 
While Ehrlich’s humoral theory has very satisfactor¬ 
ily explained the production of all varieties of anti¬ 
bodies, for both the exotoxin and the endotoxin pro¬ 
ducing bacteria, it fails to take into consideration the 
