In the mid-1980s Kärre discovered one of the mechanisms for how cells of the immune system, natural killer cells (NK cells), identify their target cells and kill them. The findings were that the NK cells are inhibited by a transplantation antigen, the major histocompatibility complex (MHC) class I, which prevents NK cells from killing their target cells. When MHC class I is removed from the target cells, they are killed by the NK cells. Kärre named this phenomenon "the missing self hypothesis".
In 1998, he was presented with the William B. Coley Award.
- Kärre, Klas (1981): On the immunobiology of natural killer cells: studies of murine NK-cells and their interactions with T-cells and T-lymphomas, Diss., Stockholm
- Kärre et al., "Selective rejection of H-2-deficient lymphoma variants suggests alternative immune defence strategy", Nature, 1986 Feb 20-26;319(6055):675-8
- The Nobel Committee 2009, accessed on September 29, 2009
- Five prominent researchers elected to the Academy, press announcement from the Royal Swedish Academy of Sciences 2009-12-16