Immunological studies on Histoplasma capsulatum.

Alveolar macrophages freshly harvested from normal and immunized rabbits were parasitized with yeast cells and protoplasts of Histoplasma capsulatum. Macrophages obtained from either normal or sensitized rabbits failed to phagocytize protoplasts, whereas, the yeast cells were actively ingested. There was no detectable intracellular killing by macrophages. A serological similarity was found between the whole yeast cell, the purified isolated cell wall, and the protoplasts of the fungus. Aprecipitin test of the protoplasts of the fungus gave a postive band, whereas immunodiffusion in agar was negative. Addition of immune sera activated phagocytosis, the immune sera against cell walls being the most active.