When blood corpuscles of an animal of a given species are injected into an animal of a foreign species the blood serum of the second animal develops the power of destroying the corpuscles of animals of the first species, that is to say, a specific hcemolytic power.
When the serum of an animal thus immunized is heated for an hour at 56° C, or when it has been allowed to stand at room temperature for twenty-four hours, it loses its hemolytic power, technically it is said to have become inactivated. It may, however, be reactivated, that is to say, its hemolytic power may be restored, by the addition of serum from another animal, - one which has not been immunized and the serum from which, therefore, does not by itself possess hemolytic power.
It is concluded from these facts that the hemolytic power of the serum of an immunized animal is dependent upon two substances: one which is chemically unstable (being easily destroyed by moderate heat or by standing at room temperature) and non-specific (being present in fresh serum of non-immunized animals as shown by reactivation), and another which is chemically stable (resisting the effect of moderate heating, etc.) and strictly specific (being present only in the serum of animals which have been immunized by injections of corpuscles). The first substance is called complement, the second amboceptor.
For specific hemolysis to occur, then, the following ingredients are required, constituting a hemolytic system: blood corpuscles+complement+hsemolytic amboceptor.
In the case of bacteria the mechanism of immunization is similar; accordingly, the essential ingredients in a reaction of specific bacteriolysis, constituting a bacteriolytic system, are: bacteria+complement +bacteriolytic amboceptor.
It was shown by Bordet and Gengou that in any bacteriolytic reaction a definite proportion of complement is used up, and that the amount of complement thus "absorbed" or "fixed" may be used as a measure of the immunity reaction. So that if upon mixing in a test-tube suspension of bacteria, complement, and bacteriolytic amboceptor we wish to determine whether bacteriolysis has taken place, we may do so simply by testing for the presence of complement; its absence would prove that it has been used up and that the immunity reaction has taken place, while its presence would prove that such reaction has not taken place.
The test for complement is done simply by adding blood corpuscles and haemolytic amboceptor; in the presence of complement haemolysis will occur, in its absence it will, of course, not occur.
The application of the phenomenon of fixation of complement with resulting inhibition of haemolysis, known as the Bordet-Gengou phenomenon, in a test for syphilis is due to Wassermann.
In the case of syphilis the ingredients of the immunity reaction are: syphilitic antigen 1 +complement+syphilitic amboceptor.
1 Antigen is a general term applied to all bodies, such as bacteria, blood corpuscles, etc., which are capable to exciting the generation of specific antibodies. The Treponema pallidum not having at that time been successfully cultivated on artificial media, Wassermann employed as syphilitic antigen watery extract of livers from congenially syphilitic infants. It has since been found that certain lipoid substances which may be extracted from normal body tissues, curiously enough, possess in a greater degree than true syphilitic antigen, the property of binding complement. Such lipoids are now exclusively employed as antigen in the reaction. It is to be judged from this that slowly, so that it usually remains good for about forty-eight hours.
The actual test is performed in two stages. In the first stage syphilitic antigen, complement, and the serum to be tested are brought together; if the serum contains syphilitic amboceptor the reaction will take place and complement will, consequently, be used up; if the serum does not contain syphilitic amboceptor the reaction will not take place and complement will therefore remain free. The second stage of the reaction consists simply in the addition of blood corpuscles and hsemolytic amboceptor to test for complement; in the case of a syphilitic serum, complement, having been used up in the first stage of the reaction, will not be available for the haemolytic system and there will be no haemolysis; in the case of a non-syphilitic serum, complement will remain free after the first stage of the test; it will therefore be available for the hsemolytic system, and haemolysis will take place.