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Journal of Agricultural Research voi. xxiii, No. 6 
resistant to tetanus for very different reasons. The first, because of the 
different chemical compositions of the brain; the second, because of 
physical incompatability to the parasite. 
Zinsser in discussing nonheritable natural immunity, says: 
The individual differences in resistance which unquestionably exist among mem¬ 
bers of the same species and races are very difficult to explain, but, as far as we can 
tell anything about them at all, they seem to depend upon variation in what is 
popularly spoken of as “general condition.” 
This indicates that the counteracting chemicals are present in greater 
or less amounts even in organisms that readily succumb to a given dis¬ 
ease. Or, stating it in another way, the particular tissue to be attacked 
can suffer injury and recover if the host is well nourished and in an 
optimum environment for its existence. This phase of natural immu¬ 
nity leads directly into the phenomenon of acquired immunity. 
Injuries or wounds in plant and animal life not only stimulate repair 
but also growth and rejuvenescence. If the injury is sufficiently severe, 
or repeated often enough, the energy required to stimulate the affected 
part is too great a tax and the organism dies. Any factor which reduces 
the energy of the organism, such as fatigue, malnutrition, unfavorable 
temperature, etc., should reduce its resistance to disease, and this is 
what actually happens. 
In naturally acquired immunity, the injury caused by the toxin of 
the invading organism stimulates an overproduction of the material 
used up in neutralizing it, so that subsequently more and more of the 
toxin can be neutralized without fatal results. Similarly in induced 
immunity, the injection of increasing amounts of toxin can be borne 
owing to the preponderantly greater amounts of neutralizing substances 
elaborated. In immunization work, one of the standard methods is to 
inject sublethal doses of fully virulent organisms in order to stimulate 
this great overproduction of these counteracting chemicals, which are 
cast into the blood and neutralize the counteracting poison at once. Such 
immunized animals may or may not, depending upon the particular an¬ 
tigen or toxin, retain this immunity for life. 
Since the beginning of modem medical sceince it has been known 
that people having once recovered from such diseases as plague, cholera, 
smallpox, or yellow fever will not again during their lifetime contract 
it; but lasting immunity is not conferred by one attack of such diseases 
as pneumonia, tetanus, or influenza. In attempting to explain or 
make use of these phenomena a great branch of physiological chemistry 
has been developed. 
The most comprehensive attempt to explain the causes of immunity 
is that offered by Ehrlich (jo), in which he conceives the cells of host 
and parasite as aggregations of complex molecules which are them¬ 
selves complex. Complex molecules react with one another through cer¬ 
tain of their side chains, but only when these side chains have a certain 
definite correspondence in structure. The reactions of immunity 
represent only a repitition of the processes of normal metabolism. A 
receptor is an outlying part (the chain) of the cell which is able to 
combine, by means of a so-called haptophorous group, with foreign 
(protein) molecules. The haptophorous group of the receptor is able 
to combine with a food molecule or with a toxin molecule. Such 
combination stimulates the cell to produce more receptors which, in 
the case of toxin combination, may result in over production, the 
