micro-organisms, changes in the
blood-plasma can be made out, though they are not associated in all
cases with changes in the formed elements which float in it, nor with
any obvious microscopical or chemical alterations.

Immunity.

The phenomena of immunity to the attacks of bacteria or their toxins, of
agglutinative action, of opsonic action, of the precipitin tests, and of
haemolysis, are all largely dependent on the inherent or acquired
characters of the blood serum. It is a commonplace that different people
vary in their susceptibility to the attacks of different organisms, and
different species of animals also vary greatly. This "natural immunity"
is due partly to the power possessed by the leucocytes or white blood
corpuscles of taking into their bodies and digesting or holding in an
inert state organisms which reach the blood--phagocytosis,--partly to
certain bodies in the blood serum which have a bactericidal action, or
whose presence enables the phagocytes to deal more easily with the
organisms. This natural immunity can be heightened when it exists, or an
artificial immunity can be produced in various ways. Doses of organisms
or their toxins can be injected on one or several occasions, and
provided that the lethal dose be not reached, in most cases an increased
power of resistance is produced. The organisms may be injected alive in
a virulent condition, or with their virulence lessened by heat or cold,
by antiseptics, by cultivation in the presence of oxygen, or by passage
through other animals, or they may first be killed, or their toxins
alone injected. The method chosen in each case depends on the organism
dealt with. The result of this treatment is that in the animal treated
protective substances appear in the serum, and these substances can be
transferred to the serum of another animal or of man; in other words the
active immunity of the experimental animal can be translated into the
passive immunity of man. According to the nature of the substances
injected into the former, its serum may be antitoxic, if it has been
immunized against any particular toxin, or antibacterial, if against an
organism. Familiar examples of these are, of the former diphtheria
antitoxin, of the latter anti-plague and anti-typhoid sera. An antitoxin
exerts its effects by actual combination with the respective toxin, the
combination being inert. It is probable that the ultimate source of the
antitoxin is to be found in the