In this series of papers it has been shown that the immune response of mice to sheep erythrocytes requires the participation of two classes of lymphoid cells. Thymus-derived cells initially react with antigen and then interact with another class of cells, the antibody-forming cell precursors, to cause their differentiation to antibody-forming cells.
Antilymphocyte globulin depressed the ability of mice to respond to sheep erythrocytes. This effect was more marked when the antigen was injected intraperitoneally than intravenously, and occurred only when the antilymphocyte globulin was given before or simultaneously with antigen. Injection of thymus cells restored to near normal the ability to respond to an intravenous injection of sheep erythrocytes. Spleen cells from antilymphocyte globulin-treated mice gave a weak adoptive immune response in irradiated recipients. The addition of thymus cells however enabled a response similar to that given by normal spleen cells.
When thymectomized irradiated recipients were used, normal spleen cells continued to give a higher response to a challenge of sheep erythrocytes at 2 and 4 wk postirradiation than did spleen cells from ALG-treated donors. This result is more consistent with the notion that thymus-derived target cells are eliminated, rather than temporarily inactivated, by antilymphocyte globulin.
These findings suggest that, in vivo, antilymphocyte globulin acts selectively on the thymus-derived antigen-reactive cells.