In summing up the results of these investigations attention is called in particular to the following facts.
Dead pneumococcus culture material does not contain the active poisons formed in infection by living pneumococci. Characteristic lesions are not induced by dead cultures. But substances are present in the pneumococcus cells, and especially in culture filtrates free from pneumococcus cells, that give rise to an immunity in which the poisons of virulent pneumococci are inactive.
In immune sera specific agglutinative, precipitative, lytic, and opsonic activities are present. But to the action of immune sera, virulent pneumococci are singularly insusceptible.
This is due chiefly to qualities acquired by the organisms during their propagation through animals. In the test-tube this insusceptibility is overcome only under exceptional conditions which destroy these qualities or neutralize their effects. Lysis may be brought about by inhibition of growth, and phagocytosis by loss of virulence. In the tissues inhibition of growth and resistance to the poisons of the pneumococcus are brought about, but in ways more subtle if less exceptional, for both lysis and phagocytosis are active factors in the recovery of certain animals from infection.