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".