The capacity to agglutinate pneumococci, to precipitate soluble specific substance, and to protect mice possessed by antipneumococcal horse serum was inhibited by the addition of methylene blue and exposure to the light of the Pointolite lamp. Agglutination and precipitation by rabbit antipneumococcal serum were also inhibited by such treatment. The native protective power of the serum of a human being against types 1, 2, and 3 pneumococci was destroyed in a similar manner. The capacity of this sample of serum (which belonged to blood group O) to clump red blood-cells of groups A and B was lost after photodynamic treatment.

Dilution of the antiserum with saline solution before exposure increased the proportion of antibody which was inactivated. The other changes studied also became proportionately more pronounced. However, when dilution was made with normal serum, the proportion of antibody which was inactivated remained approximately unchanged, thus suggesting that the reaction involving the change in antibody is a first order one, although it is not necessarily monomolecular.

Following treatment of the antipneumococcal serum from the horse with dye and light, precipitation with rabbit antihorse serum and the cutaneous reaction in sensitized rabbits were reduced. A few experiments with sensitized guinea pigs indicated a reduction in the “shocking” power of the treated serum.

An increased solubility of water-insoluble proteins of the serum followed treatment.

The active rays were absorbed by the solution of the dye.

Oxygen was considered indispensable to the reaction because destruction of antibody and other changes failed to take place in the absence of air. In the presence of an atmosphere of carbon dioxide or nitrogen the changes also failed to occur.

Certain similarities and differences between the action of ultraviolet light and photodynamic action are discussed.

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This work was made possible by a grant from Lederle Laboratories, Inc., which the author gratefully acknowledges.

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