A key activity of cellular immunity in rejection of transplants, reactions to pathogens such as viruses and tumors, is the development of T lymphocytes that specifically kill target cells. These activated cells develop during in vivo exposure or by in vitro sensitization. The CTL assay consists of increasing number of sensitized lymphocytes cultured with a fixed number of tumor or other target cells that have been prelabeled with 51Cr. To prelabel the target cells, the cells are incubated with the radiolabel 51Cr. Susequently 51Cr is taken up and reversibly binds to cytosolic proteins. When these target cells are incubated with sensitized lymphocytes, the target cells are killed and the 51Cr is released into the supernatant.

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