LYSINS 159 
iminime serum complex. If, on the other hand, the inactivated 
immune serum added to the suspension of t>-phoid bacilli be not 
typhoid immune serum, complement will not be bound to the bacteria, 
for the specific amboceptor or substance sensibilitrice will not be 
present. The complement or alexin, therefore, is not anchored to 
the bacteria, and it is free to act when the red blood cells and their 
specific inactivated serum are added to the mixture of bacteria and 
serum. Under this condition hemolysis occurs, because the red blood 
cells, inactive immune body and complement unite. The production 
of hemolysis being \isible, it serves as an indicator in such instances. 
Wassermann and his associates have utilized this method of "fixation 
of complement" for the serological diagnosis of syphilis, and gradually 
a relatively large number of diagnoses of clinical importance have been 
developed along the same lines. 
The Determination of Specific Antibodies by the Method of Complement- 
fixation. —7-* ri^cip/e Involved.— ^\hen an antigen (bacteria, erythro- 
cytes, tissue cells, protein, or other substance which stimulates specific 
antibody formation) is mixed intimately with its specific inacti\ated 
immune serum and fresh normal complement, a firm union of the 
three components takes place. ^ The result of this union is an injury 
or destruction of the antigen. If the antigen be bacterial cells or 
tissue cells there is usually no visible change in the gross appearance 
of the mixture, and cultural or chemical investigation must be relied 
upon to demonstrate the lytic process. Erythrocytes, on the other 
hand, undergo changes in the presence of inactivated specific immune 
serum and complement which result in the liberation of hemoglobin, 
which colors the solution deep red. This change is clearl>- visible 
and requires no additional procedure for its demonstration; the 
liberation of hemoglobin is in itself an indicator of the reaction which 
has taken place. 
The relation between antigen, immune serum and complement is 
(luantitative; consequently, if the respective amounts of the three 
components are correctly proportioned, no free, unattached comple- 
ment will be present in a mixture of them after an appropriate incuba- 
tion at body temperature is practised to allow of their union. I'sually 
one hour at 37° C. suffices for this union to take place quantitatively. 
These very important observations of Bordet and Gengou have 
led to the development of a technique for the diagnosis of infection, 
and the identification of antigens by the method of complement- 
fixation. 
The underlying principles of the reaction of complement-fixation 
are three : 
(a) The union of specific inactivated immune serum and homo- 
logous antigen. 
(b) The quantitative activation of the antigen— inactivated specific 
immune serum complex by non-specific complement ; and 
1 Bordet and Gengou: Ann. Inst. Pasteur, 1901, 15, 289. 
