124 IMMUNITY AND INFECTION 
or haptophore group, antl a poisoning or toxophore group. The 
former is relatively thermostabile, the latter thermolabile. If toxin 
is heated to 70° C. for a few minutes, or allowed to stand for several 
weeks, it will be found that the poisonous property of the toxin has 
disappeared, or has been materially reduced. It still retains its 
original power of uniting with aufl neutralizing antitoxin, however. 
The thermolabile toxophore group has been destroyed or weakened 
by the heating process, or on standing. The thermostabile group— 
the haptophore group— has not been impaired. Toxin which has lost 
part or all of its original poisoning properties, but which still unites 
with antitoxin is called toxoid. 
The soluble toxins of the diphtheria and tetanus bacilli are not 
simple substances; they contain at least two physiologically separate 
poisons. Thus, the toxin of the diphtheria bacillus contains in addi- 
tion to the poison which produces acute symptoms, a second poison 
which acts slowly and appears to be responsible for postdiphtheritic 
paralyses and emaciation. This second poison has less affinity for 
antitoxin than the acute poison, and it is called a toxone. Similarly, 
the tetanus toxin appears to consist of at least two distinct poisons 
— tetanospasmin, which has an especial affinity for nerve cells and 
which elicits the acute symptoms of tetanus; and tetanolysin, which 
causes hemolysis of erythrocytes. The injection of soluble or exo- 
toxins produced by bacteria leads to the formation of soluble specific 
antibodies which are called antitoxins. Antitoxins are supernumerary 
side-chains which have been produced in excess of the physiological 
needs of the cell, in response to the stimulus of a specific toxin, and 
cast off into the blood stream. 
It has been claimed that repeated injections of solutions containing 
active enzymes— as, for example, rennin — into animals, is followed 
by the appearance in the blood stream of specific antibodies which 
will prevent the activity of the homologous enzyme. These anti- 
bodies, or anti-enzymes, as they are called, exliibit the specificity and 
other characteristics which distinguish antitoxins. 
Antitoxins and anti-enzymes are called side-chains of the first 
order by Ehrlich. They possess the property of combining with and 
neutralizing their respective toxins or enzymes. 
Side-chains of the Second Order. — If substances of greater com- 
plexity than those just described are needed for the nutrition of the 
cell, some preliminary treatment, probably in the nature of digestion, 
may be required to prepare these substances for assimilation after 
they are bound to the cell. A side-chain of the first order, which 
possesses simply a combining group, does not provide the requisite 
power of digestion, according to Ehrlich, and to effect this digestion 
side-chains of somewhat more complex structure are required. Side- 
chains of this more complex type, side-chains of the second order, 
possess not only a combining group for the foodstuff, but a digestive 
group as well. This digestive or zymophore group, as it is called, 
