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Intermediate lymph (efferent from the prefemoral lymph node) was collected for 600 hr from both flanks of each of four sheep that had an autograft of skin on the left flank and a homograft of skin on the right flank. 8 days after the grafts had been applied considerable numbers of large basophilic cells, apparently identical with those that appear during immune responses to conventional antigens, appeared in the lymph draining from the homografts. No such cells appeared in the lymph draining from the autografts. At this time the homografts were already showing signs of rejection and were apparently dead well before the cellular response in the lymph reached a peak, about 350 hr (14-15 days) after the homografts had been applied. During the peak of the response up to 40% of the cells in the lymph were basophilic cells and in one experiment such cells were leaving the lymph node at a rate of 200 million per hr. Peripheral lymph (afferent to the popliteal lymph node) draining from the sites of homografts of skin was collected from five sheep. This lymph contained few white cells (<1000 per mm(3)) and showed only an insignificant lymphoid cell reaction. Although the percentage of macrophage-like cells was increased significantly there were few signs of a lymphoid cell reaction; the lymph also contained much amorphous debris. Experiments in which the basophilic cells from the efferent lymph were labeled in vitro with thymidine-(3)H and returned to the sheep by intravenous injections were carried out in six sheep. The presence of the labeled cells in the grafts, blood, and other tissue was detected by liquid scintillation counting of nucleic acid extracts of biopsy and postmortem material and by radioautography. 2-3 labeled cells out of every 1000 injected entered the homografts but hardly any entered the autografts. However, labeled basophilic cells that had originated in response to bacterial antigens entered the homografts with equal facility. It is thus hard to believe that the immunological specificity of a lymphoid cell endows it with a specific "homing" capability. Furthermore, in all the experiments the specific radioactivities of the nucleic acids extracted from the blood mononuclear cells were approximately of the same order as those of the nucleic acids extracted from the homografts. It was concluded that most of the mononuclear cells that infiltrate homografts represent a random selection from the mononuclear cell population of the blood.

Tolerance, as defined by an inability to produce runt disease and the failure to elicit a normal lymphocyte transfer reaction, was induced in Lewis and Brown Norway rats by the neonatal injection of bone marrow from the opposite strain. When thymus, thoracic duct lymph, or lymph node cells from such tolerant Lewis and Brown Norway rats were cultured together, or were individually cultured with similar cells from an F(1) hybrid of these strains, transformation was suppressed to the levels observed in nonmixed control cultures. In contrast, mixtures of cells involving nontolerant donors demonstrated significant transformation as measured by per cent enlarged cells and thymidine-(3)H uptake. The specificity of the tolerance was confirmed by the presence of transformed cells in mixtures involving cells from a tolerant donor and an unrelated or a partially related third strain of rats. From these results, it is suggested that the mixed lymphocyte reaction may be used as a simple test for tolerance and that it most likely represents an in vitro immune response.

Efferent lymph of the popliteal lymph nodes of rabbits was collected 4 days after a single footpad injection of SRBC. Thin-layer agar plating was done to isolate plaque-forming cells of the lymph for electron microscope examination, and the numbers of plaque-forming cells (PFC) in cells from the lymph and lymph nodes were determined. Of 71 PFC of lymph isolated and examined, 93% were lymphocytes, most of them with signs of substantial levels of physiologic activity. The cytoplasm showed an abundance of free ribosomes and many finger-like projections. The endoplasmic reticulum (ER) was barely detectable in most of the active lymphocytic PFC, and in some, a few short narrow channels of ER could be seen. Approximately one-fifth of the lymphocytic PFC presented an appearance of senescence, with signs of degeneration: rounded cells, with amorphous nuclear chromatin, and very few microvilli. The remaining 7% of the PFC of the lymph showed an unusual combination of features: small round cells with a narrow ring of cytoplasm which, however, contained well-organized channels of ER. Such cells had been found only among PFC of peripheral blood of the rabbit. The number of PFC per million cells was higher in the lymph than in the suspensions of lymph node cells. In both the contralateral lymph node and its efferent lymph, the number of PFC was less than 1% that of the injected side.

Plaque-forming cells (PFC) in the regional lymph node, its efferent lymph, and the spleen of rabbits immunized with BCG were enumerated by localized hemolysis in gel, using sheep red blood cells coated with tuberculin protein. In the primary response, the maximal level of PFC production was reached at 5 days in the regional lymph node, and at 7 days in the spleen, whereas efferent lymphatic cells showed only a small number of PFC. Sharp peaks of PFC levels at 3 days were seen in the regional lymph node, spleen, and the efferent lymphatic cells in the secondary response. Blast cell outflow from the regional lymph node was maximal at 3 days in the secondary response as well as in the primary response. The relation between blast cell outflow and PFC production was discussed.