ecific antibody is always formed in greater or lesser amount;
and in experimental work a sufficient amount of any required antibody
can often be obtained without carrying the process of immunisation to
its logical termination.

For instance, if the immunisation of a rabbit toward Bacillus typhosus
is commenced on the lines already set out it will often be found, after
a few injections of "killed" cultivation that the blood serum of the
animal (even when diluted with several hundred times its volume of
normal saline) contains specific agglutinin for B. typhosus--and if the
sole object of the experiment has been the preparation of agglutinin the
inoculations may well be stopped at this point, although the animal is
not yet immune in the strict meaning of the word.

Again, antibodies may be formed in response to antigens other than
infective particles--thus the injection into suitable animals of foreign
proteins such as egg albumin, heterologous blood sera or red blood discs
from a different species of animal, will result in the formation of
specific antibodies possessing definite affinities for their respective
antigens.

The most important antibody of this latter type is Haemolysin, a
substance that makes its appearance in the blood serum of an animal
previously injected with washed blood cells from an animal of a
different species. The serum from such an animal possesses the power of
disintegrating red blood discs of the variety employed as antigen and
causing the discharge of their contained haemoglobin, and is specific in
its action to the extent of failing to exert any injurious effect upon
the red blood cells of any other species of animal.

The action of this serum is due to the presence of two distinct bodies,
complement and haemolysin.

_Complement_ (or alexine) is a thermo-labile readily oxidised body
present in variable but unalterable amount in the normal serum of every
animal. It is a substance which exerts a lytic effect upon all foreign
matter introduced into the blood or tissues; but by itself is a
comparatively inert body, and is only capable of exerting its maximum
lytic effect in the presence of and in combination with a specific
antibody, or immune body.

Complement is obtained (unmixed with antibody) by collecting fresh blood
serum from any healthy normal (that is uninoculated) animal.
Guinea-pigs' serum is that most frequently employed for experimental
work.

_Haemolysin_ (immune body, copul