. Probably the lining
endothelium of the blood-vessels as well as other tissues of the body
participate in the production of anti-substances.

[Sidenote: Natural immunity.]

The subject of artificial immunity has occupied a large proportion of
bacteriological literature within recent years, and our endeavour has been
mainly to indicate the general laws which are in process of evolution. When
the facts of natural immunity are examined, we find that no single
explanation is possible. Natural immunity against toxins must be taken into
account, and, if Ehrlich's view with regard to toxic action be correct,
this may depend upon either the absence of chemical affinity of the living
molecules of the tissues for the toxic molecule, or upon insensitiveness to
the action of the toxophorous group. It has been shown with regard to the
former, for example, that the nervous system of the fowl, which possesses
immunity against tetanus toxin, has little combining affinity for it. The
non-sensitiveness of a cell to a toxic body when brought into immediate
relationship cannot, however, be explained further than by saying that the
disintegrative changes which underlie symptoms of poisoning are not brought
about. Then as regards natural powers of destroying bacteria, phagocytosis
aided by chemiotaxis plays a part, and it can be understood that an animal
whose phagocytes are attracted by a particular bacterium will have an
advantage over one in which this action is absent. Variations in
chemiotaxis towards different organisms probably depend in natural
conditions, as well as in active immunity, upon the opsonic content of the
serum. Whether bacteria will be destroyed or not after they have been
ingested by the leucocytes will depend upon the digestive powers of the
latter, and these probably vary in different species of animals. The blood
serum has a direct bactericidal action on certain bacteria, as tested
outside the body, and this also varies in different animals. Observations
made on this property with respect to the anthrax bacillus at first gave
the hope that it might explain variations in natural immunity. Thus the
serum of the white rat, which is immune to anthrax, kills the bacillus;
whereas the serum of the guinea-pig, which is susceptible, has no such
effect. Further observations, however, showed that this does not hold as a
general law. The serum of the susceptible rabbit, for example, is
bactericidal to this organism, whilst th