Cell kinetic studies in mixed leukocyte cultures: an in vitro model of homograft reactivity.

The recognition phase of homograft immunity can be studied in vitro in the mixed leukocyte culture reaction. In this reaction, at the end of seven days, up to 30 per cent of the lymphocytes in culture "respond" to a single allogeneic cell stimulus. This paper presents evidence that the lymphocytes responding in culture divide with the generation time of 18-21 hours, with some asynchrony, and that a large percentage of the cells found at the end of the culture period may be the products by division of a small number of cells initially responding. Two estimates are made of the frequency of the initially responding unit (the cell(s) responsible for initiating the response). These are probably minimum estimates and are in the range of 1 in 200 to 1 in 2000 cells which can initially respond. This very high frequency of responding units as compared with the responding units in immediate type hypersensitivity (antibody production) is discussed.