By replacing one of the H atoms in the l)enzol ring with the methyl 
radical (CHg) we have toluol; 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 hydrox}d radical and another one witii 
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 alEnities, to which the molecule of 
protoplasm is likened. 
In applying this analogy to the molecule of 
protoplasm the name receptor” is given these 
side chains, or secondaiy atomic complexes of 
the molecular group. Contraiy to the simple 
analogies above given each molecule of proto- 
plasm has maiy^ different kinds of receptors, as 
shown by the schematic diagram in fig. \a. These receptors have a 
specific affinity for the molecules of food, and also combine with the 
toxic molecules. 
Fig. la * — The cell with its 
various combining groups 
or side chains, known as 
receptors. Various toxines 
are shown having specific 
affinity for the proper 
shaped receptors. 
The toxi?i molecule, according to Ehilich, consists of two important 
parts. One is known as the toxophore group ^ the other as the liapto- 
phore group). 
The toxop)hore group of the toxin is that portion of the molecule 
which exerts a poisonous effect upon the protoplasm of the cell. This 
group is less stable than the haptophore 
group. 
The haptop)hore group) is 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 
certain cells, which in part explains the 
selective action of these poisons. 
Toxines, such as diphtheria toxine, 
graduall}^ diminish in toxiciHg 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 0.003 c. c. to kill a 
guinea pig. After nine months this poison weakened, so that it 
Wo.pt'opln 
(^roup 
or<L 
To)(,ophore 
Fig. lb . — The to.nn molecule; showing 
the haptophore (combining) group, 
and the toxophore (poison) group. 
* Foot Note. — Fig. la-i. — Diagrammatic representation of Ehrlich’s side chain theory of immunity 
(Croonian Lecture, Proc. Royal Society of London, vol. 66, 1900, p. 437). 
