408 Dr. J. Mellanby. [June 3, 
Diphtheria antitoxin belongs to the class (6), composing about 85 per cent. 
of the total proteins of serum. 
The similarity of diphtheria antitoxin to the main mass of the protein of 
serum raises the question as to the function of these proteins. 
It is generally assumed that the main function of the proteins of serum is 
to nourish the tissues. From a general consideration this 1s improbable. 
Foodstuffs are divided into three great classes :—proteins, fats, and carbo- 
hydrates. Sugar is present in blood to the extent of about 0-2 per cent., and 
usually only a trace of fat can be detected in it. It is improbable, therefore, | 
that proteins to the extent of 10 per cent. are circulating in the blood to be 
used as food. 
From a study of the processes involved in the coagulation of blood it is 
fairly clear that globulin originates from a splitting of the fibrinogen molecule 
into fibrin and globulin when coagulation occurs. Albumen (8) composing 
12 per cent. of the total protein may possibly be regarded as concerned in 
the nutrition of the tissues. Since diphtheria antitoxin belongs to the protein 
group composing the remaining 85 per cent. of protein, it may be legitimate 
to assume that a function of these proteins is to protect the tissues from 
disease in addition to maintaining the equilibrium of electrolytes in the 
tissues, and holding carbon dioxide in the blood. The assumption of sucha 
function would afford an explanation why large injections of normal horse 
serum after control diseases caused by bacterial infections. 
THE RELATION BETWEEN THE ANTITOXIC POTENCY OF A SERUM AND THE 
AMOUNT OF PROTEIN CONTAINED IN IT, 
The relation between the antitoxic power of a serum and the solids con- 
tained in it has been investigated by various workers. 
Szontagh and Wellmann (6) found only a small increase in the total solid 
present in serum after immunisation against diphtheria toxin. 
Hiss and Atkinson (7) found a rise of globulin content during the process 
of immunisation and a corresponding fall in the albumen content. The 
globulin was determined by precipitation with magnesium sulphate. 
Ledingham (8), using half-saturation with ammonium sulphate as a precipi- 
tant, also found an increase of globulin content during immunisation. 
The work of Hiss and Atkinson, and Ledingham, was based on the assump- 
tion that diphtheria antitoxin was a globulin as defined by its precipitation 
limits with ammonium sulphate and magnesium sulphate. But from a con- 
sideration of the influence of acids and alkalis on the precipitation of the 
proteins of serum by neutral salts, it is clear that their results could be 
