Summary A radioactive antigen which emitted gamma rays was prepared by chemical attachment of radioactive iodine, I131, to crystallized hen egg albumin. The rates of disappearance of radioactivity from sites of injection were determined with a Geiger-Mueller counter shielded with lead to give directional collimation. The labelled antigen was specifically precipitated with antiiodinated ovalbumin serum as well as with anti-ovalbumin serum. An increased retention of radioactivity after intradermal injection of the iodinated ovalbumin persisted only in those rabbits which were sufficiently immune to iodinated ovalbumin to respond with a pronounced inflammatory (Arthus) reaction. Tracer radioactive albumin was also retained to a greater degree than in normal skin when injected into a site undergoing an Arthus reaction produced by the simultaneous injections of bovine gamma globulin into animals sensitized to this antigen. This increased retention of ovalbumin was interpreted as a non-specific effect of inflammation since no antiovalbumin was present.
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