weight have again returned to
normal--generally about seven days after the inoculation--again inject
killed cultivation, this time giving a dose of 5 c.c. intravenously and
20 c.c. intraperitoneally. A temperature and weight reaction similar to,
but less marked than that following the first injection will probably be
observed, but after about a week's interval the animal will be ready for
the next injection.

10. When ready to give the third injection prepare a fresh blood agar
subculture from another O.C. tube and after twenty-four hours incubation
prepare a minimal lethal dose (as determined in 5) and inject it
subcutaneously into the rabbit's abdominal wall.

A slight local reaction will probably be observed as well as the weight
and temperature reactions.

11. A week to ten days later inject a similar minimal lethal dose into
the peritoneal cavity.

12. Observe the weight and temperature of the rabbit very carefully, and
regulating the dates of inoculation by the animal's general condition,
continue to inject living cultivations of the pneumococcus into the
peritoneal cavity, gradually increasing the dose by multiples of ten.

13. At intervals of two months samples of blood may be collected from
the posterior auricular vein and the serum tested for specific
antibodies.

14. Under favourable conditions it will be found after some six months
steady work that the rabbit may be injected intraperitoneally with an
entire blood agar cultivation without any ill effects being apparent;
and this characteristic--resistance to the lethal effects of large doses
of the virus--is the sole criterion of _immunity_. Further, the serum
separated from blood withdrawn from the animal about a week after an
injection, if used in doses of .01 c.c., will protect a mouse against
the lethal effects of at least ten minimal lethal doses of living
pneumococci.

In the foregoing illustration it has been assumed that complete acquired
active immunity has been conferred upon the experimental rabbit in
consequence of the formation of antibody, specific to the diplococcus
pneumoniac, sufficient in amount to ensure the destruction of enormous
doses of the living cocci--the _antigen_ (that is the substance injected
in response to which _antibody_ has been elaborated) in this particular
case being the bacterial protoplasm of the pneumococcus with its
endo-toxins.

But provided death does not immediately follow the injection of the
antigen, sp