STUDIES ON EXPERIMENTAL PNEUMONIA : V. ACTIVE IMMUNITY AGAINST EXPERIMENTAL PNEUMOCOCCUS PNEUMONIA IN MONKEYS FOLLOWING VACCINATION WITH LIVING CULTURES OF PNEUMOCOCCUS.

1. The subcutaneous injection of small doses of living virulent Pneumococcus Type I stimulates in monkeys a degree of active immunity sufficient to protect them against experimental pneumococcus pneumonia of homologous type. 2. The subcutaneous injection of living avirulent Pneumococcus Type I, if administered in a sufficiently large dose, likewise renders the monkey immune to a subsequent pneumonia of homologous type. 3. Vaccination of monkeys with small doses of living virulent pneumococci may or may not be followed by a severe constitutional reaction, depending on the natural resistance of the individual. The severe reactions are caused by the development of a pneumococcus septicemia. which is either temporary, or leads to a fatal termination. The mild reactions are not accompanied by septicemia, and there are no symptoms other than a slight elevation of temperature and moderate leucocytosis. Vaccination with living avirulent pneumococci does not induce severe reactions and is not accompanied by pneumococcus septicemia. 4. Active immunity against pneumococcus pneumonia, produced by vaccination with living pneumococci, appears to be largely independent of the presence or absence of agglutinins and protective bodies in the serum of the monkey. 5. Vaccination with living cultures of Pneumococcus Type I confers against other types of pneumococci a certain amount of cross-immunity which, however, varies considerably with the individual monkey. 6. immunity against pneumococcus, like other forms of immunity, is a relative term, and depends upon the capacity of the individual for antibody production, the virulence of the invading microorganism, and the size of the dose injected.