152 ANTIGENS AND THE TECHNlC OF SERUM REACTIONS 
If a specific organism is added to an appropriate dilution of unknown 
serum with proper precautions, and characteristic chnnping takes 
place, or if a known specific serum is added with suitable precautions 
to a suspension of an unknown organism, and characteristic clumping 
takes place, a specific diagnosis of the serum and consequently of 
infection, or of the organism can be arrived at. In the first instance, 
a diagnosis of a disease may be made ; in the second instance the identity 
of an organism may be established. The laboratory diagnosis of 
typhoid and paratyphoid fever, of the various types of bacillary 
dysentery and of other bacterial infections is frequently made by 
testing the serum of the patient for agglutination with a known 
culture of the organism. ^ The laboratory identification of specific 
bacteria, conversely, is frequently established or corroborated through 
their agglutination with known specific agglutinating sera. 
The reaction of agglutination may be made either microscopically 
or macroscopically, 
1. Microscopic Method.— A drop of serum from a patient, diluted 
to the proper degree, is mixed with an equal amount of a broth culture 
of the desired organism on a clean cover-glass,^ and then suspended 
over the cavity of a hollow ground slide, ringed with vaseline to 
prevent evaporation, and examined under the microscope. Motile 
bacteria, as for instance B. typhosus, soon lose their motility (immo- 
bilization) and gradually collect in small groups which tend to coalesce 
into larger and larger clumps, leaving the field between them practically 
free from organisms. The bacteria are not necessarily killed by aggluti- 
nation. The reaction ordinarily is complete within one to two hours. 
Recently Mudd^ has used the "resuspension method" of Gaehtgens^ to 
demonstrate agglutination in suspensions of bacteria which are difficult 
to work with using the ordinary technique. The principle involved is 
to centrifuge the bacteria after exposure to appropriate serum, and 
then resuspend the sediment ed bacteria, together with proper controls, 
by shaking. The intensity of the reaction is indicated by the size of 
the resulting clinnps, the controls being uniformly dispersed throughout 
the solution. The technical details, which are very simple, will be 
found in Mudd's article. Killed cultures of bacteria may be used in 
place of living cultures but the reaction is usually less clear-cut. 
The advantage of the microscopic method lies chiefly in the small 
amount of serum required to perform the test. One of its chief 
disadvantages lies in the relative inaccuracy of the dilution of the 
serum. (See chapter on B. typhosus for full discussion of technic.) 
' The technic and precautions to be observed are discussed individually in the chap- 
ters upon Specific Pathogenic Bacteria. 
" For a majority of bacteria, eighteen-hour cultures grown in 0.1 per cent, glucose 
broth are particularly advantageous. Cultures grown in plain broth are usually much 
less actively motile and agglutinate somewhat less readily. 
3 Jour. Immunol., 1927, 13, 113. 
* Mlinchen. med. Wchnschr., 1906, 53, 1351; Arch. f. Hyg., 1908, 46, 377. 
