ff and Bordet subsequently devised means by which a similar change
could be produced _in vitro_, and analysed the conditions necessary for its
occurrence. It has been completely established that in this phenomenon of
lysogenesis there are two substances concerned, one specially developed or
developed in excess, and the other present in normal serum. The former
(_Immunkoerper_ of Ehrlich, _substance sensibilisatrice_ of Bordet) is the
more stable, resisting a temperature of 60deg C., and though giving the
specific character to the reaction cannot act alone. The latter is
ferment-like and much more labile than the former, being readily destroyed
at 60deg C. It may be added that the protective power is not lost by
exposure to the temperature mentioned, this apparently depending upon a
specific anti-substance. Furthermore, lysogenic action is not confined to
the case of bacteria but obtains also with other organized structures,
_e.g._ red corpuscles (Bordet, Ehrlich and Morgenroth), leucocytes and
spermatozoa (Metchnikoff). That is to say, if an animal be treated with
injections of these bodies, its serum acquires the power of dissolving or
of producing some disintegrative effect in them. The development of the
immune body with specific combining affinity thus presents an analogy to
antitoxin production, the difference being that in lysogenesis another
substance is necessary to complete the process. It can be shown that in
many cases when bacteria are injected the serum of the treated animal has
no bacteriolytic effect, and still an immune body is present, which leads
to the fixation of complement; in this case bacteriolysis does not occur,
because the organism is not susceptible to the action of the complement. In
all cases the important action is the binding of complement to the
bacterium by means of the corresponding immune body; whether or not death
of the bacterium occurs, will depend upon its susceptibility to the action
of the particular complement, the latter acting like a toxin or digestive
ferment. It is to be noted that in the process of immunization complement
does not increase in amount; accordingly the immune serum comes to contain
immune body much in excess of the amount of complement necessary to
complete its action. An important point with regard to the therapeutic
application of an anti-bacterial serum, is that when the serum is kept _in
vitro_ the complement rapidly disappears, and accordingly the complement