AGf~ICULTURAL RESEARCH, PUSA.

28 PRACTICAL BACTERIOLOGY
in the preparation of diphtheria and tetanus
antitoxins by immunising animals-e.g. horses
-with the respective toxins. '1'0 ensurc that
no injuriolls effects may result from immunisation,
toxin which has bccn rendered non-toxic
(toxoid) by certain chemicals-e.g. iodine, formalin,
etc.-or by heat, or has been neutralised
by antitoxin, is frequently used. Immunisation
with diphtheria toxin has been applied in
the prophylaxis of diphtheria in the human
subject, but to obviate any harmful results it
is necessary to introduce it along with antitoxin
(toxin-antitoxin immunisation); toxoid is now
generally uscd in preference to toxill (vide
p.289).
The serum of an actively immunised animal is designated
an irnrnlLne serurn or antiserurn, and owes its
effect to specific antibodies or immune bodies which act
adversely on the homologous organism or neutralise its
toxins.
The special principle in the organism or its toxin
which incites antibody production is termed an
antigen.
A serum containing antibodies antagonistic to the
particular bacterium is spoken of as an antibacterial
serurn; one which contains antibodies that neutralise
toxin, as an antitoxic serurn.
It has to be noted that, apart from bacteria and
their toxins, other cells, and in fact all soluble complete
protein substances, may act as antigens and
incite specific antibody production - e.g. red blood
corpuscles, blood serum, animal venoms, etc.
Thus the red corpuscles of one species injected into
an animal of different species incite the development of
an antibody which, under certain conditions, can effect
lysis of the red cells of the former. This antibody is
described as a haernolysin and the serum containing
it as a haernolytic antiserum.
As a genera] rule, an antigen, to produce immunity,
must be injected parenterally-i.e. by some route