Rats irradiated with 600 r (total body) showed increased indices of both opsonophagic and surface phagocytosis 2, 12, and 24 hours after irradiation. The indices decreased 72 hours after exposure and returned to normal by the 4th to 5th postirradiation day.

Bactericidal power of the blood of irradiated animals was depressed when measured 3 and 6 days after exposure to x-rays.

Decomplemented serum from irradiated animals was more bactericidal against M. aureus than decomplemented serum from the control rats. This bactericidal substance was destroyed at 78°C. and was found not to require any complement or specific antibodies for action.

Extracts of leucocytes from animals 3 days after irradiation demonstrated no bactericidal activity against M. aureus, while extracts from the control animals or rats 1 day after irradiation were actively bactericidal. The extracts from leucocytes of non-irradiated animals were thermolabile, non-dialyzable, had a pH optimum of 7.5, and were capable of being precipitated by NH4SO4 and of calcium phosphate fractionation.

The observed increase in susceptibility of irradiated rats to infection with M. aureus was correlated not only with granulopenia but also with the alteration of the above indicated functions of the granulocytes of the irradiated animals.

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