Abstract
Rabbit anti-mouse lymphocyte serum (ALS) was capable of depressing delayed (24- and 48-hour) footpad reactions to sheep erythrocytes in mice previously sensitized with sheep erythrocytes in adjuvant. The degree of depression depended on the mode of preparation of the ALS, the dose, and the time of injection relative to the footpad test. After intraperitoneal injection of ALS there occurred: lymphopenia in blood and peritoneal fluid, the appearance of blast cells in peritoneal fluid, and a temporary loss of macrophages followed by the appearance of larger and ultrastructurally changed macrophages in the peritoneal fluid; temporary loss of macrophages also followed the injection of normal rabbit serum. The ability of different batches of ALS to depress delayed footpad reactions could not be correlated exactly with any of the other changes or with the titre of antibodies to lymph node or peritoneal lymphocytes as measured by immune adherence. Delayed footpad reactions to allogeneic lymphocytes in mice sensitized by skin allografts were depressed by ALS given just before footpad testing. In mice which had combined early (Arthus) and delayed-type hypersensitivity to sheep erythrocytes, ALS treatment also depressed the early component of the footpad reactions.