INFECTION AND IMMUNITY. 
41 
of which is always present in the blood serum, the 
other only after immunization has occurred. This 
latter substance will act only with the substance to 
which the serum is immune and for this reason is said 
to be specific. These three substances taking part in 
the solution or hemolysis of the red blood cells are 
designated, antigen, complement and amboceptor. 
In the example given above the antigen is the 
human red blood cell, the complement is the substance 
always present in the blood serum and the amboceptor 
is the substance present in the blood serum of the rab¬ 
bit after immunization. 
The term antigen is applied to any substance 
which, when injected into a living animal, causes the 
formation of antibodies, viz., red blood cells, bacteria 
or bacterial poisons. The immune substance produced 
by the injection of the antigen is called the amboceptor. 
It differs from the complement that is present in all 
serum by the fact that it is not so sensitive to heat and 
so is said to be thermostabile. 
In the experiment just described with the human 
red cells and the rabbit’s serum immunized to them, it 
is possible to add just enough corpuscles to use up or 
fix all of the complement. 
Similar experiments may be made with a number 
of antigens, such as the gonococcus, the treponema pal¬ 
lidum, the typhoid bacillus, the glanders bacillus and 
others. This principle of mixing antigen, complement 
and amboceptor in definite proportions so that the com¬ 
plement is fixed is the basis of complement fixation as 
