Purified tritiated tetanus toxin was injected intraperitoneally into 42 mice previously immunized with tetanus toxoid. An equivalent amount of tritiated tetanus toxin neutralized with mouse antitoxin was injected intraperitoneally into 28 nonimmunized mice. Animals were autopsied at various intervals, and autoradiograms made of the peritoneal exudate, spleen, bone marrow, lymph nodes and thymus.

  1. Neutrophils containing radioactive antigen were observed in all animals autopsied between 2 and 24 hr after injection, following which there was a marked reduction in number. Many of the neutrophils were phagocytized by macrophages.

  2. Lymphocytes and macrophages containing radioactive antigen were present in the inflammatory exudate 2 hr after injection. In the immunized animals injected with tritiated toxin, there was a progressive increase in the number of macrophages containing antigen, up to the 4th day. In the nonimmunized animals injected with neutralized toxin fewer labeled macrophages were observed, and there was a progressive decrease in their number after the 1st day of inflammation.

  3. Labeled eosinophils were observed between the 2nd and 8th days of inflammation. The eosinophils did not engulf antigen directly but appeared to receive it from disintegrating labeled mononuclear cells.

  4. Certain mononuclear cells became swollen, highly vacuolated, and acted as a center around which other cells became attached, forming a rosette of cells. The number and cellular composition of these rosettes varied at different stages of inflammation.

  5. Macrophages and basophilic mononuclear cells containing radioactive material were observed in the spleen, mediastinal, inguinal, axillary, mesenteric and cervical lymph nodes. These cells were also observed in the bone marrow and occasionally in the thymus.

  6. Consistently greater numbers of eosinophil and mononuclear cells containing radioactive material were found in the inflammatory exudate and lymphoid tissues of animals which had been immunized to tetanus toxoid prior to the injection of the radioactive toxin.

These results indicate that antigen localizes in different cells during various stages of an inflammatory response, but eventually becomes concentrated in macrophages. It was suggested that the role as well as the fate of the antigen in a macrophage varies depending upon the manner in which it reaches the macrophage.

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