The study investigates the role of histocompatibility determinants in the interaction between macrophages and T lymphocytes in antigen recognition. The authors demonstrate that the activation of immune thymus-derived lymphocytes by antigen-pulsed macrophages requires the histocompatibility match between the macrophage and the lymphocyte. This interaction can be blocked by alloantisera, suggesting that it is mediated by histocompatibility antigens or their products. The findings indicate that the macrophage is the initial cell to interact with antigen and present it to the T lymphocyte, and that this interaction is crucial for T-lymphocyte proliferation. The study also shows that the requirement for histocompatible macrophages is not specific to the antigen used and is consistent across different species.The study investigates the role of histocompatibility determinants in the interaction between macrophages and T lymphocytes in antigen recognition. The authors demonstrate that the activation of immune thymus-derived lymphocytes by antigen-pulsed macrophages requires the histocompatibility match between the macrophage and the lymphocyte. This interaction can be blocked by alloantisera, suggesting that it is mediated by histocompatibility antigens or their products. The findings indicate that the macrophage is the initial cell to interact with antigen and present it to the T lymphocyte, and that this interaction is crucial for T-lymphocyte proliferation. The study also shows that the requirement for histocompatible macrophages is not specific to the antigen used and is consistent across different species.