4 THOMPSON YATES LABORATORIES REPORT 
B. enteridttis. But there is generally some difference, and any given serum acts best, viz., in greater 
dilution, on its ow^n kind of bacillus, so that it can be used for its identification without causing 
confusion. 
The extent to which any serum can be diluted and yet show an effect depends in part on 
the virulence of the culture employed ; the more attenuated the culture, the weaker the serum 
solution necessary to produce an effect. It is to this grouping power that Durham gave the name 
' clumping action ' of the serum, and the hypothetical body producing it was termed by Gruber 
'agglutinin.' 
It is obvious that the reaction can be applied to the identification of various kinds of bacilli 
if we possess specimens of the corresponding ' immune ' sera ; and perhaps, as will be indicated 
later, it may be of use in the discovery of new forms. In spite, therefore, of the sources of error 
just mentioned, a 'typhoid' or 'cholera' serum can prove of great use in determining whether 
doubtful organisms belong to these species. It is also clear that the reverse will be true : that by 
means of a known bacillus we can identify an unknown serum, and can with comparative ease 
ascertain whether an animal is at all, or to what extent, protected. 
That the ' agglutinin ' is distinct from the bactericidal and paralyzing substances in 
' immune ' or other serum appears from the following considerations. An ' immune ' serum 
exhibiting strong agglutinative power need not be appreciably more bactericidal than normal 
serum. A normal serum may exhibit agglutinative without paralyzing action {']a). Agglutinins 
are not produced in the serum of animals immunized against certain micro-organisms, e.g., 
B. diphtherdce and certain micrococci. 
Motility of the organism is not, as was at first supposed, essential for agglomeration. Not 
only can agglutination occur using dead, motionless bacilli instead of active, living ones {2b\ but it 
may be seen also with some non-motile bacteria and cocci. With some cocci I have seen a 
phenomenon resembling the loss of movement of motile bacteria. Before the addition of the 
serum the cocci usually exhibit a Brownian movement, which is much restrained or even entirely 
inhibited by the action of suitable serum (7//). 
Gruber explains the phenomenon of agglomeration by the supposition that the agglutinin 
causes the enclosing membranes of the bacilli to swell out and become sticky, and that they 
consequently adhere to each other when they come in contact. He also put forward the idea 
that the effect of the special agglutinin is thus to facilitate the action of the general bactericidal 
substances common to all sera ; and his theory of immunity is based on these two fundamental 
ideas. 
The theory has not been unanimously accepted. Pfeiffer considers each serum to have 
a strictly specific action. Salimbeni does not believe agglutination to occur inside the body (12). 
Nobody has been able, as yet, to demonstrate a swelling of the capsule. Be all this as it may, the 
observed facts, without any theory, form a valuable addition to bacterial knowledge. 
The ' active ' immunization of animals is usually attained by the injection, either hypo- 
dermically or into the peritoneal cavity, of dead or attenuated cultures of the micro-organisms. 
The destruction of the culture used may be effected by the use of either heat or disinfectants, but 
