13 
Fig. la.* — The cell wTth its 
various combining groups 
or side cbaius. kno^vn as 
recejjtors. Various toxines 
are shown having specific 
aflinity for the proper 
shaped receptors. 
replacing one of the H atoms in the benzol ring with the methyl 
radical (CH 3 ) we have toluol; by replacing one of the H atoms with 
the hydroxyl group (OH) we have phenol: by substituting two 
hydroxyl groups we have resorcin, etc. : three, pyrogallic acid. etc. ; 
by substituting one hydrogen atom of the ring 
with the hydroxyl radical and another one with 
the methyl radical we have the cresols. 
These simple illustrations from well known 
organic compounds illustrate the central mole- 
cular mass of atoms with its side chains and 
combining affinities, to which the molecule of 
protoplasm is likened. 
In apphung this analogy to the molecule of 
protoplasm the name "receptor" is given these 
side chains, or secondary atomic complexes of 
the molecular group. Contrary to the simple 
analogies above given each molecule of proto- 
plasm has many diherent kinds of receptors, as 
shown by the schematic diagram in lig. la. These receptors have a 
specific affinity for the molecules of food, and also combine with the 
toxic molecules. 
The toxi/i molecule, according to Ehrlich, consists of two important 
parts. One is known as the toxopliore groups the other as the hapto- 
phore group). 
The toxopjhore groiqj of the toxin is that portion of the molecule 
which exerts a poisonous eflect upon the protoplasm of the cell. This 
group is less stable than the haptophore 
group. 
The haptopjhore groupie the seizing or 
combining portion of the toxin molecule 
(anroD^ to seize or attack). The hapto- 
phore group of the toxins have specific 
combining affinities for the receptors of 
ceitain cells, which in part explains the 
selective action of these poisons. 
Toxines. such as diphtheria toxine. 
gradually diminish in toxicity, but retain 
the same power of chemical combination with the antitoxin. This 
phenomenon is explained by the formation of toxoids. 
Ehrlich inferred the presence of the toxoid from the following sim- 
ple experiment: He had a toxine which required 0.M03 c. c. to kill a 
guinea pig. After nine months this poison weakened, so that it 
Houp-toplic 
"ToK.ophor€ 
Fig. 1&.— The toxin molecule; showing 
the haptophore (eomhining) group 
and the toxophore (poison) group. 
* Foot Note.— Fig. la-t. — Diagrammatic representation of Ehrlich's side chain theory of immunity 
(Croonian Lecture, Proc. Royal Society of London, vol. 66, 1900, p*. 4b<). 
