Antigen-antibody Reaction: Nature of Complex Initiating Delayed Hypersensitivity.

Two homologous lightly coupled dinitrophenyl conjugates of poly-L-lysine of differing average molecular sizes were compared with regard to their abilities to elicit in guinea pigs specific delayed hypersensitivity skin reactions, passive cutaneous anaphylaxis, and active Arthus reactions. Equal concentrations by weight (but not equimolar concentrations) of the two conjugates elicited equally intense delayed hypersensitivity reactions and Arthus reactions, whereas equimolar concentrations (but not equal weightconcentrations) elicited equally intense passive cutaneous anaphylaxis reactions. These results suggest that delayed hypersensitivity reactions are initiated by the reaction of antigen with antibody molecules in true solution, and not by the simple bridging by antigen of a small number of antibody molecules firmly fixed to cell membrane surfaces. Whether "sensitized cells" or circulating "delayed hypersensitivity antibodies" are the specific mediators of the delayed hypersensitivity reactions is discussed.