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conferred is as great as that given by other vaccines which contain the bodies of
the bacteria and lasts as long.

       Other methods of preparing anti-typhoid vaccine, such as that described by
Macfadyan and Rowland, have been promulgated from time to time, but as far
as we know all vaccines at present in common use are obtained by one of the
methods described above.

       With this introduction we may now pass on to the question of the standar-
disation of these several vaccines, a question which is generally recognised
to be of dominating importance. The first desideratum in this connection is of
course to place before ourselves a perfectly clear mental picture of what is
connoted by the term standardisation. It seems to us that Wright⁷ has supplied
us with an excellent definition of the term. He says, ' Standardisation involves,
on the one hand, the determination of the amount of immunising element
contained in a standard volume of the vaccine material; on the other hand, the
adjustment of the dose administered to the resisting powers of the man or
animal subjected to the inoculation.' If we accept this definition, it is evident
that the first problems which have to be settled before any scientific method of
standardisation can be evolved are problems concerning the nature of the
immunising element of the vaccine and the manner in which it accomplishes
its object. Let us for a moment consider these questions.

       It will, we think, be acknowledged by all pathologists that the immunity
induced by anti-typhoid vaccination is in the main, if not entirely, a bactericidal
immunity, that is to say, that it depends on an increased power of the individual
to kill the typhoid bacteria by means of his body fluids, and that there is no
evidence so far of an antitoxic immunity. This increased power is due, using the
language of Ehrlich, to an increase of the specific amboceptor or immune body
in the body fluids and not to any increase of the complement.

       It is also probable that the organism, after an anti-typhoid inoculation,
acquires a habit of producing these protective substances on a stimulus of a
much slighter nature than before the inoculation. Such an hypothesis would
account for the presence of a certain degree of immunity after the amount of
protective substances, which are increased considerably in quantity immediately
after an inoculation of anti-typhoid vaccine, had again fallen to normal.

       It is, however, possible that we have also to deal with the action of the
substances called by Wright ' opsonins ' or of the substances named by
Metchnikoff 'stimulins'. But the proof of these substances taking any part
in the process of typhoid immunity is still lacking. We need not, therefore,
take them into account at the present moment. We have consequently only to
account for an increased production of immune body which is called forth as a
result of the inoculation of the immunising substances contained in the vaccine.

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