Question

why are NK cells suck an important componet of the immune system?

Answer 1

Innate immune system:
Natural killer cells play a major role in defending the host by providing rapid responses to virally infected cells and tumor formation. NK cells have the ability to distinguish between normal and uninfected cells from infected cells and tumors by recognizing the absence of antibodies and MHC class I presented on the surface of the infected cells and destroy the altered cells by releasing cytotoxin granules. NK cells do not require activation for killing cells which are missing MHC 1 markers. (Other immune cells cannot detected or destroy the cells that are missing MHC class1, which is why this role of NK cells is especially important).

Adaptive immune system:
Natural killer cells develops antigen-specific long-lasting immunological memory, basically for responding to secondary infections with same antigen.