1908. ] Diphtheria Antitoxin. 409 
adequately explained by an assumption of varying alkalinity of the serum 
during immunisation. 
It has been shown in the previous pages that diphtheria antitoxin 
possesses the same physical properties as nine-tenths of the proteins of serum, 
and that this antitoxic protein is probably added to serum during the process 
of immunisation. The question arises as to whether the theory of the 
addition of antitoxic protein to the blood during the process of immunisation 
recelves any support from a consideration of the total solids 1 in the blood of a 
horse during its antitoxic life. 
A detailed examination of the quantity of protein in the sera of various 
horses showed that there was no relation between the total proteins in the 
blood of a horse and its capacity to produce antitoxin. From the antitoxic 
point of view we may consider the proteins present in the serum of an 
animal as .concerned in two great functions: (a) The maintenance of 
tissue equilibrium; (0) the protection of the tissues. Of these two divisions 
(a) is the more active component, and would be more evidently reflected in 
the amount of protein contained in the serum. The absence of any relation 
between the antitoxic producing powers of different animals and their 
general metabolism reflects the experience of individuals—that a strong 
physical condition does not imply a special capacity to resist disease. 
But the question whether the percentage of solids in the blood of a horse 
at different times of its antitoxic life bears any relation to the quantity of 
antitoxin in it is of a different nature. In this case it appears reasonable to 
assume that, so long as we do not interfere with the general nutrition of the 
animal by poisoning with excess of toxin, the percentage of solid in the serum 
will increase with its antitoxic power. To test the accuracy of this assump- 
tion, the percentages of solid in the sera of a number of horses at different 
periods of their antitoxic life were determined. 
In the following table (p. 410) the days are reckoned from the commence- 
ment of immunisation. 
From the results we may deduce two generalisations :— 
(a) That at the start of diphtheria toxin immunisation the percentage of 
solid increases with the antitoxic power. 
(6) That after a time, depending on the particular horse, there is no 
connection between the percentage of solid in the serum and its 
antitoxic power. This stage is marked by a large decline in the 
antitoxic producing power of the horse. 
These two generalisations are readily appreciated if we consider the protein 
in the serum of a horse to reflect the two functions stated above: (i) the 
maintenance of tissue equilibrium; (ii) the protection of the tissues. The 
