Contact sensitivity skin reactions were produced in mice by immunization with 2-phenyl-4-ethoxymethylene oxazolone (oxazolone) and detected by the increase in ear thickness after challenging the ears with 2% oxazolone. These skin reactions can be transferred from immunized donors to irradiated recipients by peritoneal exudate cells induced by thioglycollate.

The peritoneal exudate cells were separated into purified macrophage and purified lymphocyte populations. Both cell populations transferred skin reactions. However, their time course was different. The reactions produced by lymphocytes were greater at 24 hr than at 12 hr while the reactions produced by macrophages declined slightly between 12 and 24 hr.

The working hypothesis was formed that the peritoneal lymphocytes conveyed a factor (presumptive cytophilic antibody) to peritoneal macrophages which enabled them to transfer ear reactions. Experiment showed that peritoneal and lymph node lymphocytes from sensitized donors within a Millipore chamber conveyed a factor to macrophages outside the chamber which enabled them to transfer ear reactions. In contrast, peritoneal macrophages (from sensitized donors) within the chamber and peritoneal lymphocytes outside the chamber were inactive.

These findings suggested that there are three modes of immunological tissue damage: hypersensitivity mediated by lymphocytes (classical delayed hypersensitivity), hypersensitivity mediated by circulating antibody (classical immediate type hypersensitivity), and hypersensitivity mediated by macrophages which have passively acquired a factor (macrophage-mediated hypersensitivity).

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