The functional activity of the reticuloendothelial system (RES), as determined by the intravascular clearance of colloidal carbon, and the degree of induced RE hyperplasia were studied following the intravenous injection of various constituents of zymosan. The readily extractable lipid component from the yeast cell wall was inactive, as was the polysaccharide, mannan. Stimulatory activity was still present in the zymosan residue after removal of free and bound lipids. The administration of glucan derived either from yeast, or its cell wall, resulted in marked RE activation and induced hyperplasia, demonstrating it to be the active RE stimulating agent. The relationship of glucan's chemical structure to its ability to induce RE hyperfunction and hyperplasia is discussed.

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