Sera taken from monkeys recently recovered from severe yellow fever infections possess a precipitin capable of reacting with a precipitinogen which occurs in the blood of monkeys during the period of acute illness. This precipitinogen is not the virus of yellow fever, but appears to be associated with a protein of the albumin fraction. Its concentration reflects the severity of illness. It disappears with recovery, after stimulating the formation of a precipitating antibody. This resulting precipitin is entirely independent of the protective antibody resulting from an infection.

A similar precipitin occurs in the serum of humans recently recovered from a severe yellow fever infection. This precipitin reacts with the precipitinogen occurring in the blood of monkeys during the acute phase of illness.

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