PRECIPITINS-PRECIPITINOIDS 153 
2. Macroscopic Method. — Various dilutions of serum, accurately 
measuretl by \()liuiu'tric pipettes, are brought into small, sterile 
test-tubes, together with suspensions or broth cultures of the bacteria. 
Agglutination is manifested by the gradual accumulation of a floccu- 
lent sediment of bacteria, leaving the supernatant liquid perfectly 
clear. Control tubes without serum remain uniformly clouded. 
The part played by agglutinins in immunity is unknown; the 
concentration of agglutinins in immune sera, as measured by present- 
day methods, throws no light upon the degree of immunity or the 
prognosis. Very severe typhoid infections, for example, may show 
little agglutinin in their sera, and mild cases may exhibit sera com- 
paratively rich in agglutinin content. Their chief value at the present 
time lies in their relation to the diagnosis of disease. 
PRECIPITINS. PRECIPITINOIDS. 
In the preceding section it was shown that the sera of animals 
immunized with various bacteria contained substances— agglutinins 
—which agglutinated the specific organisms. Kraus^ showed that 
these immune sera would cause a precipitate when they were added 
to clear filtrates of the specific organisms. During the process of 
immunization, therefore, specific antibodies, termed jjrecipitins , are 
formed, which react with the specific soluble antigen, yreciyitinogen, 
in germ-free filtrates of broth cultures of the specific organisms, to 
form a precipitate. Dale^ has pointed out the resemblances between 
the precipitin reaction and anaphylaxis. Later investigations have 
shown that any soluble protein, as egg-albumen, injected into experi- 
mental animals may stimulate the production of specific precipitins 
wdiich appear in the animal's serum, and which will cause a precipita- 
tion in clear solutions of the homologous protein. 
According to Eisenberg,^ such a reaction comes to an equilibriimi, 
and in the supernatant fluid both antigen and antibody co-exist 
without combining. The equilibrium may be disturbed by the addition 
of either antigen or antibody. This phenomenon is more commonly 
met with in complex soluble proteins, as egg-white or blood serum. ^ Von 
Dungern^ indeed explained it upon the well-known fact that serum, 
for example, really contains several substances, each of which acts as 
an independent antigen. These various antigenic substances are 
present in varying concentrations in the serum and each induces 
specific antibody formation. Consequently each has its own antibody 
in the serum of the immune animal. When, therefore, the homologous 
serum is added to its antiserum, one antibody may imite completely 
1 Wien. klin. Wchnschr., 1897, 10, 7.36. 
2 Proc. Roy. Soc, vol. 91, B. 126. 
3 Centalbl. f. Bakteriol., Abt. I, 1902, 31, 773. 
* Wells (Jour. Infec. Dis., 1911, 9, 147) states that there are five distinct antigens in 
egg-white. 
'- Centalbl. f. Bakteriol., Abt. I, orig., 1903, 34, 355. 
