r this reason, considers that
agglutinins are coagulative ferments.

The phenomenon of agglutination depends essentially on the union of
molecules in the bacteria--the agglutinogens--with the corresponding
agglutinins, but another essential is the presence of a certain amount of
salts in the fluid, as it can be shown that when agglutinated masses of
bacteria are washed salt-free the clumps become resolved. The fact that
agglutinins appear in the body at an early stage in a disease has been
taken by some observers as indicating that they have nothing to do with
immunity, their development being spoken of as a reaction of infection.
This conclusion is not justified, as we must suppose that the process of
immunization begins to be developed at an early period in the disease, that
it gradually increases, and ultimately results in cure. It should also be
stated that agglutinins are used up in the process of agglutination,
apparently combining with some element of the bacterial structure. In view
of all the facts it must be admitted that the agglutinins and immune bodies
are the result of corresponding reactive processes, and are probably
related to one another. The development of all antagonistic substances
which confer the special character on antimicrobic sera, as well as
antitoxins, may be expressed as the formation of bodies with specific
combining affinity for the organic substance introduced into the
system--toxin, bacterium, red corpuscle, &c., as the case may be. The
bacterium, being a complex organic substance, may thus give rise to more
than one antagonistic or combining substance.

[Sidenote: (c) Opsonic action.]

By opsonic action is meant the effect which a serum has on bacteria in
making them more susceptible to phagocytosis by the white corpuscles of the
blood (_q.v._). Such an effect may be demonstrated outside the body by
making a suitable mixture of (a) a suspension of the particular bacterium,
(b) the serum to be tested, and (c) leucocytes of a normal animal or
person. The mixture is placed in a thin capillary tube and incubated at
37deg C. for half an hour; a film preparation is then made from it on a
glass slide, stained by a suitable method and then examined
microscopically. The number of bacteria contained within a number of, say
fifty, leucocytes can be counted and the average taken. In estimating the
opsonic power of the serum in cases of disease a control with normal serum
is made at the same time