Structure of Freund's complete and incomplete adjuvants. Relation of adjuvanticity to structure.

Emulsions of complete (CFA) and incomplete (IFA) Freund's adjuvants were examined in the light and electron microscopes, and the resulting morphological findings were correlated with the effectiveness of the emulsions as immunological adjuvants. Thick (viscous) emulsions of both IFA and CFA consisted of highly stable, three-dimensional meshworks composed of interconnecting strands of antigen-containing water droplets interspersed in oil phase. Included mycobacteria were confined to this meshwork and were coated with an adherent surface layer of water droplets. Thin Freund's adjuvants were less stable, relatively coarse emulsions, but even in such preparations mycobacteria showed a striking affinity for the surface of water droplets when these contained low concentrations of antigens such as human serum albumin (HSA). The characteristic adjuvant effect of CFA was observed only when associations between mycobacteria and water droplets took place. Thus, no adjuvant effect occurred with oil-in-water (o/w) emulsions, nor when antigen and mycobacteria-in-oil were injected into separate foot pads. Further, a good adjuvant effect was observed even with thin emulsions when mycobacteria-water droplet associations were abundant. These morphological and immunological data suggest that CFA is a device for bringing extrinsic, water-soluble antigens into intimate, stable contact with myco-bacteria, thereby conferring on them the ability to elicit an immunological response qualitatively similar to that induced by mycobacteria-in-oil to the intrinsic antigen, tuberculin.