A delayed-type hypersensitivity reaction initiated by a single T lymphocyte.

In vivo primed T cells injected into the footpad of naïve recipients elicit a typical delayed-type hypersensitivity (DTH) reaction in the presence of their specific antigen. Serial dilutions of primed T cell populations were used in order to score positive and negative transfers. The values for the footpad swelling obtained after these transfers followed a bimodal distribution. This clear bimodal distribution with no overlapping between positive and negative transfers suggested that a single cell initiates the DTH reaction. Limiting dilutions of cloned T cells transferred with their antigen allowed us to demonstrate that a single cell is able to transfer a specific DTH reaction.