a, sensitising body, amboceptor) is a
_thermostable_ antibody formed in response to the injection of red cells
which although in itself inert is capable of linking up complement
present in the normal serum to the red cells of the variety used as
antigen--a combination resulting in haemolysis.

Haemolysin is obtained by collecting fresh blood serum from a suitably
inoculated animal and exposing it to a temperature of 56 deg. C. (to
destroy the thermo-labile complement) for 15 to 30 minutes before use.
It is then referred to as _inactivated_, and is _reactivated_ by the
addition of fresh normal serum--that is serum containing complement.

Haemolysin is of importance academically owing to the fact that many of
the problems of immunity have been elucidated by its aid; but its
present practical importance lies in the application of the _haemolytic
system_ (that is haemolysin, corresponding erythrocyte solution and
complement) to certain laboratory methods having for their object either
the identification of the infective entity or the diagnosis of the
existence of infection.

For use in these laboratory methods of diagnosis it is most convenient
to prepare haemolytic serum specific for human blood--whether the
laboratory is isolated or attached to a large hospital. Ox blood, sheep
blood or goat blood if readily obtainable, may however be used instead,
and although the following method is directed to the preparation of
human haemolysin the same procedure serves in all cases.

THE PREPARATION OF HAEMOLYTIC SERUM.

_Apparatus Required:_

Small centrifuge, preferably electrically driven, with two
receptacles for tubes, and enclosed in a safety shield (Fig. 162).
Sterile centrifuge tubes (10 c.c. capacity), Fig. 163.
Sterile pipettes (10 c.c. graduated) in case.
Sterile glass capsules (in case).
Sterile test-tubes.
Sterile all glass syringe (5 c.c. or 10 c.c. capacity)
and needle.

[Illustration: FIG. 162.--Small electrical centrifuge.]

[Illustration: FIG. 163.--Centrifuge tube.]

_Reagents Required:_

Normal saline solution.
10 per cent. sodium citrate solution in normal saline.
Human blood (_vide infra_).

METHOD.--

1. Select a healthy full-grown rabbit of not less than 2500 grammes
weight in accordance with the directions already given (page 322) and
prepare it for intraperitoneal inoculation.

2. Measure out 2 c.c. citrated human blood (collected at a surg