158 
Dourine 
Brief Explanation. 
The general principles and mechanism of the complement fixation 
reaction are now so widely known that it seems unnecessary for the 
purposes of this paper to repeat them in detail, a few remarks on the 
subject sufficing to make it clear and intelligible. 
When an antigen is introduced into an animal either by way of natural 
infection or by artificial administration a group of reaction products 
arise in the animal’s serum—known as antibodies—bearing a specific 
relationship to the antigen and able to combine with it outside of 
the animal body under certain conditions. Micro-organisms, foreign 
blood cells and sera, albumens and many forms of protein matter are 
able to act as antigens. Thus an animal infected with dourine produces 
antibodies resulting from and specifically related to the dourine antigen, 
namely Trypanosoma equipetdum, the actual cause of the disease. 
In a similar manner, an animal which has received injections of 
the blood of another animal species becomes possessed of antibodies 
having a specific affinity for the blood of that particular species of animal. 
In other words, the antibody arising in response to the exciting antigen 
in the process of infection, sensitization, or immunization, has the 
specific function of acting upon that antigen to neutralize it or prepare 
it for destruction. 
The complement fixation test applied in the diagnosis of disease 
consists of two sets of antigen and antibody, that is, two distinct and 
separable combining groups having no relationship to one another but 
in each of which Complement —a constituent of normal serum—is an 
essential factor. It is convenient to distinguish these groups by referring 
to the one comprising haemolytic serum, red cells and complement as 
the ‘haemolytic system’ and to the other—closely related to the disease 
—comprising dourine antibody, corresponding antigen, and complement 
as the ‘antibody-combining group.’ Thus: 
Before the test can actually be applied the exact dosage of the 
different elements in each group must be worked out by careful quanti¬ 
tative titrations—the most important step in the whole proceeding—- 
and the operator must be absolutely assured that each group reaction 
