The fate of 14C-labeled allergens injected intradermally into guinea pigs, namely picryl chloride (PCl*) and 2:4 dinitrochlorobenzene (DNCB*), was followed during the induction period of delayed hypersensitivity. Both chemicals were applied in a single injection into one ear in amounts that approached their minimal sensitizing doses (PCl, 0.25 µg; DNCB, 5.0 µg). Radioactivity in the various tissues was determined by liquid scintillation counting after combustion of tissues to CO2 and H2O.

The injected allergens seemed to leave the injection site in three phases. A large proportion of allergen escaped rapidly from the ear, about 50% within 3 hr in the case of PCl, within 15 min for DNCB, the difference probably reflecting their unequal reaction constants. Initially there was a "half-life" escape in 2.5 hr with injected dosage of 0.25 µg PCl and in 18 hr for 5.0 µg DNCB. This escape occurred via the regional veins and not via the lymphatics. Radioactive decomposition products of the allergens were already present in the urine within 3–4 hr.

After 6–8 hr, the half-life time of escape lengthened to approximately 28 hr for both allergens used in their respective initial dosages, holding up to 2 days after which there occurred still further slowing; between 2 and 4 days the time was about 43 hr for PCl, much longer (72–88 hr?) for DNCB, apparently reflecting different physicochemical properties of this second fraction. Sensitization seemed to be connected with the portion that was present between 12 hr and 4 days of the induction period. It is not known how far the escape of radioactivity during this period may represent gradual hydrolysis of attached picryl and dinitrophenyl groupings, respectively, to form picric acid and dinitrophenol. Gradual accumulation of the second fraction in the regional lymph nodes could definitely be excluded. It was noted that no hypersensitivity arose and essentially no depot of radioactivity existed between 12 hr and 4 days when DNCB was injected in a dose of 0.25 µg, owing to its ready escape from the ear; but 20 times as much DNCB caused sensitization and provided about the same fixed depot as 0.25 µg of picryl chloride.

After delayed hypersensitivity had been established, traces of radioactivity were still measurable at the site. This third fraction, probably representing a different coupling product, escaped at a very low rate and was traceable up through several weeks.

No demonstrable radioactivity could be detected in thymus, spleen, and mesenteric nodes when examined at short intervals between ½ min and 17 days.

In analogy with findings on transplantation "immunity" and with studies reported in the following paper, the induction of delayed hypersensitivity can be explained by encounters between lymphoid cells and the hapten complex which is found present in the local site for 4 days, in agreement with Medawar's concept of peripheral sensitization.

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