Ultrastructural Study of Plasmodium knowlesi Antigen Used in Vaccination of Rhesus Monkeys*

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Abstract
Material obtained from various steps of the French pressure cell technic of preparing antigen from P. knowlesi-infected red cells was examined by EM. A positively charged colloidal Fe solution was used to differentiate between membranes of host red cells and parasites. Red cell membranes take up the stain, whereas parasite membranes do not. This antigen, which was previously used to protect monkeys against P. knowlesi, appears to consist almost entirely of membrane-bound vesicles. Some of these vesicles contain a fine granular material, whereas others appear empty. The antigen failed to stain with the positively charged Fe solution, which suggests that it is free of contamination by host cell membrane.