In case of cytotoxic T lymphocytes, it has been demonstrated that at a single cell level a mouse effector lymphocyte can kill one target and has the potential to recycle and kill up to six targets in a sequential manner
http://plosone.org/article/info:doi/10.1371/journal.pone.0000326#article1.body1.sec1.p1

Unfortunately, we did not cite the original work that actually showed serial killing by CTL. Here are the references which should have been included:

Berke, G., Sullivan, K.A. and Amos, D.B. (1972). Tumor immunity in vitro: Destruction of a mouse ascites tumor through a cycling pathway. Science, 177: 433-434.

Zagury, D., Bernard, J., Thierness, N., Feldman, M. and Berke, G. (1975). Isolation and characterization of individual functionally reactive cytotoxic T lymphocytes. Conjugation, killing and recycling at the single cell level. Eur. J. Immunol., 5: 818-822.