A method was developed for the quantitative determination of opsonins for C. burnetii.

An opsonin unit for C. burnetii was defined as that quantity of opsonin which, under standardized conditions, causes phagocytosis of elementary bodies of C. burnetii by 94 to 100 per cent of polymorphonuclear neutrophils. The opsonin titer represents the number of opsonin units in 0.1 ml. of serum or heparinized plasma.

A plasma opsonin titer of 1 occurred in 1 per cent of the people tested at Camp Detrick, Frederick, Maryland, with no known contact with C. burnetii; in 30 per cent of the immunized people of Camp Detrick treated by subcutaneous injection of phenol-inactivated C. burnetii in chick embryo yolk sac; and in 31 per cent of the people tested at Los Angeles, with no clinical evidence of Q fever.

On a statistical basis, about 42 per cent of the vaccinated people of Camp Detrick and of "normal" Los Angeles people had evidence of increased opsonins in their blood.

Vaccinated individuals of Camp Detrick showed no relation between opsonin and complement fixation titers, but individuals living in Los Angeles, an area endemic for Q fever, revealed positive complement fixation tests only in one-seventh of individuals with opsonin titers of 1. The significance of these differences is discussed.

The opsonin titer of patients with Q fever may increase many million fold, and much more than the complement fixation titer. The opsonin titers of serum and heparinized plasma were identical, and both decreased with storage at 5°C.

The opsonin titer for elementary bodies of C. burnetii grown in chick embryo yolk sac may be used as an epidemiological and diagnostic indicator for the distribution of the agent causing Q fever.

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