living cells of the tissues and that it
passes from them into the blood. The action of an antibacterial serum
depends on the presence in it of a substance known as "immune-body,"
which has a special affinity and power of combining with the bacterium
used. In order that it may exert this power it requires the presence of
a substance normally present in the serum known as "complement." The
development of these "anti-bodies," though it has been studied mainly in
connexion with bacteria and their toxins, is not confined to their
action, but can be demonstrated in regard to many other substances, such
as ferments, tissue cells, red corpuscles, &c. In some animals, for
example, the blood serum has the power of dissolving the red corpuscles
of an animal of different species; e.g. the guinea-pig's serum is
"haemolytic" to the red corpuscles of the ox. This haemolytic power
(haemolysis) can be increased by repeated injections of red corpuscles
from the other animal, in this case also, as in the bacterial case, by
the production and action of immune-body and complement. The antiserum
produced in the case of the red corpuscles may sometimes, if injected
into the first animal, whose red corpuscles were used, cause extensive
destruction of its red corpuscles, with haemoglobinuria, and sometimes a
fatal result.

Opsonic action depends on the presence of a substance, the "opsonin," in
the serum of an immunized animal, which makes the organism in question
more easily taken up by the phagocytes (leucocytes) of the blood. The
opsonin becomes fixed to the organisms. It is present to a certain
extent in normal serum, but can be greatly increased by the process of
immunization; and the "opsonic index," or relation between the number of
organisms taken up by leucocytes when treated with the serum of a
healthy person or "control," and with the serum of a person affected
with any bacterial disease and under treatment by immunization, is
regarded by some as representing the degree of immunity produced.

Agglutinative action is evidence of the presence in a serum of a
somewhat similar set of substances, known as "agglutinins." When a
portion of an antiserum is added to an emulsion of the corresponding
organism, the organisms, if they are motile, cease to move, and in any
case become gathered together into clumps. In all probability several
different bodies are concerned in this process. This reaction, in its
practical applications at least, may