72 
PRINCIPLES OF BACTERIOLOGY 
responding side chains for a certain exotoxin, in the 
first place, he will never be infected with this kind of 
exotoxin—natural or inherited immunity. 
From his observations Ehrlich concluded that the 
toxin molecule consists of two parts: the haptophore 
group (the “carrying” part) which unites with the 
side chain of the body cell, and the toxophore group 
(the “poisoning” part), which does the actual dam¬ 
age (Fig. 12). 
Fig. 12.—Ehrlich’s conception of a toxin molecule. The toxin molecule 
(cd) consists of two parts: c, which combines with the receptor or side- 
chain b and is called the haptophore (the carrying or anchoring) group, and 
d, which contains the poisonous part and is called the toxophore group. 
Some of the poisonous substances are attached di¬ 
rectly to the side chain, as we have seen in the example 
of the diptheria exotoxin, such side chains, as for ex¬ 
ample, antitoxins, are called receptors of the first order. 
Such side chains as agglutinins and precipitins are 
called receptors of the second order and differ from 
the antitoxins in that they have not only the combin¬ 
ing group but also a digesting group (because the ag¬ 
glutinins and the precipitins not only unite with bac¬ 
terial substances, as the antitoxins do with exotoxins, 
but also cause clumping and precipitating; hence the 
need of the digesting group). (Fig. 13). 
