AGGLUTININS, AGGLUTINOIDS AND PROAGGLUTINOIDS 151 
trated serum is known as a "proagglutinoid" reaction; it is attributed 
by Ehrlich to the presence of "agglutinoids" in the serum — side-chains 
of the second order which ha\e k^st their agghitinophore group, but 
still retain their combining group (haptophore group). These "agglu- 
tinoids," which are deteriorated agglutinins, have a greater affinity 
for the agglutinogen of the bacteria than have the unchanged agglu- 
tinins, and consequently pre\'ent the latter from becoming attached 
to the organisms. If the serum is diluted sufficiently, a point is reached 
where the agglutinoids are numerically too few to interfere with the 
action of the agglutinins, which usually far outnumber the agglutin- 
oids. As the serum is more and more diluted a point is eventually 
reached where the content of agglutinin is insufficient to react with 
the bacteria. This phenomena of prozone as shown above (page 150), 
may be reproduced by adding substances obtained from the autolysis 
of bacteria, or certain media constituents.^ Whether the increased 
viscosity of the agglutinating serum under these circumstances is the 
cause of the non-agglutination of the bacteria is not as yet determined. 
If, however, bacteria are cultured in this dilute serum, they frequently 
develop into long, thread-like, interwoven filaments, the so-called 
"thread-reaction" of Pfaundler. It is obvious that the maximum dilu- 
tion at which a serum will agglutinate bacteria depends somewhat 
upon the number of organisms; there is, in other words, a quantitative 
relation between the amount of agglutinin in the serum and agglutino- 
gen in the bacteria. 
Non-agglutinable Bacteria.— Occasionally strains of bacteria, as 
B. typhosus, freshly isolated from the body, may not agglutinate 
with the specific serum. This resistance to agglutination is supposed 
to result from some unknown change in the agglutinogen of the bac- 
terium during its development in the body. However, inagglutinable 
bacteria may adsorb agglutinins.- A similar loss of agglutinability 
may be experimentally brought about by growing the bacteria in 
gradually increasing concentrations of specffic agglutinating serum 
outside the body. This inagglutinability is usually lost after a few 
days' development on artificial media; the organisms will then clump 
in a characteristic manner in a serum that originally was ineffectiA'e. 
On the other hand, inagglutinable strains may arise spontaneously 
from perfectly typical organisms.^ 
The Reaction of Agglutination.— The practical value of the reaction 
of agglutination depends upon the visible clumping or agglomeration 
of a suspension of bacteria in a fluid medium containing some neutral 
salt, when a relatively small amount of immune serum specific for 
the organism is added to it. The reaction may be expressed thus: 
Organism (Agglutinogen) + Specific Serum (Specific Agglutinin) 
= Agglutination. 
1 Krumwiede, Cooper and Provost: Jour. Immunol., 1925, 10, 93. 
2 Buchanan: Jour. Bacteriol., 1919, 4, 73. 
' See Hadley: Microbic Dissociation, Jour. Infec. Dis., 1927, 40, 117, for full discus- 
