By the immuno-electron microscopy of Toxoplasma gondii, it was shown that ferritin particles were deposited densely on the limiting membrane of the Toxoplasma-containing vacuole of the host cell. The organism itself was not labeled with ferritin as long as it was located in an intact vacuole.
Antibody, although able to infiltrate into the infected host cell, is arrested at the limiting membrane of the vacuole and cannot come into contact with the organisms. This is the reason why the Toxoplasma in the host cell remains always negative by the Sabin-Feldman dye test.
Particulate precipitates usually seen in the vacuole have an antigenic property as evidenced by the ferritin labeling. Therefore, they are of parasitic origin. These findings confirmed our previous observation that the Toxoplasma cyst wall was produced by the accumulation of Toxoplasma substances on the limiting membrane of the vacuole in which Toxoplasma were located.