e serum of the immune dog is not.
In the case of the latter animal the serum [v.03 p.0180] contains an
opsonin which leads to phagocytosis of the bacillus, and the latter is then
destroyed by the leucocytes. It is quite evident that bactericidal action
as tested _in vitro_ outside the body does not correspond to the degree of
immunity possessed by the animal under natural conditions. We may say,
however, that there are several factors concerned in natural immunity, of
which the most important may be said to be the three following, viz.
variations in the bactericidal action of the serum _in vivo_, variations in
the chemiotactic or opsonic properties of the serum _in vivo_, and
variations in the digestive properties of the leucocytes of the particular
animal. It is thus evident that the explanation of natural immunity in any
given instance may be a matter of difficulty and much complexity.

AUTHORITIES.--Bacteriological literature has become so extensive that it is
impossible to give here references to original articles, even the more
important. A number of these, giving an account of classical researches,
were translated from French and German, and published by the New Sydenham
Society under the title _Microparasites in Disease: Selected Essays_, in
1886. The following list contains some of the more important books
published within recent years. Abbott, _Principles of Bacteriology_ (7th
ed., London, 1905); Crookshank, _Bacteriology and Infective Diseases_ (with
bibliography, 4th ed., London, 1896); Duclaux, _Traite de microbiologie_
(Paris, 1899-1900); Eyre, _Bacteriological Technique_ (Philadelphia and
London, 1902); Fluegge, _Die Mikroorganismen_ (3rd ed., Leipzig, 1896);
Fischer, _Vorlesungen ueber Bakterien_ (2nd ed., Jena, 1902); Guenther,
_Einfuehrung in das Studium der Bakteriologie_ (6th ed., Leipzig, 1906);
Hewlett, _Manual of Bacteriology_ (2nd ed., London, 1902); Hueppe,
_Principles of Bacteriology_ (translation, London, 1899); Klein,
_Micro-organisms and Disease_ (3rd ed., London, 1896); Kolle and
Wassermann, _Handbuch der pathogenen Mikroorganismen_ (Jena, 1904)
(supplements are still being published; this is the most important work on
the subject); Loeffler, _Vorlesungen ueber die geschichtliche Entwickelung
der Lehre von der Bacterien_ (Leipzig, 1887); McFarland, _Text-book upon
the Pathogenic Bacteria_ (5th ed., London, 1906); Muir and Ritchie, _Manual
of Bacteriology_ (with bibliography, 4th ed., Edin. and