A Theory of Ferments and Their Action. 
37 
show that opsonic phenomena, so-called, are the phenomena of immu- 
nity acquired against pathogenic bacteria, and that opsonin is a new 
name for an old acquaintance 1 — alexin. 
The serum of persons who have been made immune to a species of 
pathogenic bacteria is bactericidal to this species ; the bactericidal action 
is specific. When this serum is heated to 55° C., it loses its bactericidal 
power, but this can be restored to it by the addition of fresh serum, 
either native or alien. This shows that immune serum must contain 
two substances on which its bactericidal power depends, and that one 
of these is destroyed by heating to 65° C. These two substances are 
called alexin, or complement, and immune body. Alexin is a normal 
constituent of serum; it may enter into chemical combinations with 
certain elements or their ions; it is non-specific in its action, and may 
be destroyed by heating to 55° C. It appears, therefore, that the char- 
acteristics of alexin are identical with those of opsonin. The other sub- 
stance contained in the serum of persons made immune to a species of 
pathogenic bacterium is named immune body; this is not a normal 
constituent of serum; is found only in immune or partially immune 
serum; is specific in its action; is not destroyed by heating to 55° C. ; 
responds to no revealing chemical test, and is a proteid in its structure 
and a ferment in its action. 
Now, since the bactericidal power of serum depends on the presence 
and combined action of both alexin and immune body, the nature of 
this relation should be pointed out. Ehrlich’s side-chain theory affords 
a chemical explanation, but the one that most appeals to me is one that 
interprets the reciprocal action of alexin on immune body to be that of 
an activator on a ferment. 
Enzymes, which play an important role in the phenomena of life, 
have their activities conserved and regulated to the needs of the organ- 
ism by the stimulating or restraining action of bodies the organism 
contains that are called activators and hormones. For example, hydro- 
chloric acid, secreted by the glands of the stomach, is an activator that 
converts inactive pepsinogen into active pepsin; inactive tryp si nogen 
is converted into active trypsin by entero-kinase, its activator; while the 
amount of trypsin secretion is regulated by secretin, its hormone. These 
are a few of many similar function regulators by means of which the 
organism co-ordinates and controls functions which otherwise may be- 
come dangerous. From my viewpoint immune body is an activator that 
stimulates into activity the ferment action of alexin, which destroys 
the bacteria. It is not necessary to assume that the increased number 
of bacteria that phagocytes take in when inoculations of a vaccine are 
made is due to an increase in the opsonic content of the serum, since 
it has been fully established that the number of a species of bacteria 
