Question

why do we have both NK cells and CTL that both target cancerous and virally infected cells ?

Answer 1

NK cells are mostly known for killing of tumor cells as well as virus infected cells. They do not need any priming by antigen presenting cellsas required in cytotoxic t cells which also take part in killing of virus infected cells and cancer cells. Nk cells are always on a natural patrol and when they sense cells with activating signals, they respond to them .

Normal cells have MHC I receptors on their surface which marks them as self and priduce inhibitory signals for Nk cells to have a check on them. Whereas cancer and virus infected cells lose the mhc 1 receptors being vulnerable to nk cells which further release perforins and granzymes for the killing of target cells.

Cytotoxic t lymphocytes kill the target cells by releasing cytokines and making way to apoptosis.