The capacity of the serum of rabbits following intradermal pneumococcus infections to precipitate in the presence of pneumococcus C polysaccharide has been studied during the resultant periods of active infection and during recovery. In rabbits infected with Type I, III, or VIII pneumococci, large hemorrhagic lesions are produced which frequently bring about death of the animals after a febrile illness of 3 to 4 days. Repeated precipitation tests with the sera of these animals have been uniformly and consistently negative, not only during the acute illness but in the recovery period as well.

On the other hand, the sera of monkeys of the Macacus cynomolgos species actively ill with experimental Type III pneumonia have been shown to react in precipitation tests with the C substance. The serum reaction appears within the first 24 hours after infection, remains positive in high titer for 2 to 3 days during the acute illness, and disappears with the onset of recovery. The precipitation reaction with C also occurs with the sera of monkeys following intradermal and intraperitoneal infection with pneumococci. The results of precipitation tests of the serum of monkeys during experimental pneumonia are similar to those obtained with the sera of patients suffering from pneumococcus lobar pneumonia.

From the results of these studies it would appear improbable that the demonstration of the serum precipitation phenomenon with C polysaccharide in monkeys, and possibly also in man, is conditioned by previous exposure to pneumococcus antigen.

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