Suppression of delayed hypersensitivity by the depletion of circulating monocytes.

The intraperitoneal injection of Marcol oil was previously reported to result in the depletion of circulating monocytes. The way in which this depletion affected guinea-pigs' ability to respond to challenge with antigen was tested by challenging with tuberculin after sensitization and examining the ability to respond to DNCB. It was found that the ability to respond to tuberculin was completely abrogated by the injection of Marcol and could be restored by removing the oil from the peritoneal cavity. The ability to respond to a secondary challenge with DNCB was not affected, while the ability to become sensitized was impaired. A normal inflammatory response was seen. The role of the circulating monocyte is discussed.