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Natural Killer (NK) cells are cytotoxic lymphocytes of the innate immune system, critical for early defense against viral infections and cancer. Unlike T cells, they do not require prior sensitization. NK cells identify and kill target cells that have downregulated MHC class I molecules—a common immune evasion tactic by tumors and viruses—through a “missing-self” recognition mechanism, inducing apoptosis via perforin and granzymes.
Natural Killer (NK) cells provide a rapid, first-line defense against cellular threats. Their function is governed by a balance of signals from activating and inhibitory receptors on their surface. Healthy cells express MHC class I molecules, which are recognized by inhibitory receptors on NK cells (like KIRs and NKG2A). This interaction sends a strong “don’t kill me” signal, preventing the NK cell from attacking. This is known as “missing-self” recognition: the absence of the ‘self’ MHC class I signal is what triggers the attack.
When a cell becomes cancerous or is infected by certain viruses, it often reduces the expression of MHC class I molecules to evade detection by cytotoxic T cells. This, however, makes it a prime target for NK cells. The lack of an inhibitory signal, often coupled with the presence of stress-induced ligands recognized by activating NK cell receptors (like NKG2D), tips the balance towards activation. Once activated, the NK cell’s cytotoxic mechanism is very similar to that of a CTL. It releases perforin and granzymes to induce apoptosis in the target cell. NK cells can also induce cell death through other pathways, such as the Fas/FasL interaction, and are key players in antibody-dependent cell-mediated cytotoxicity (ADCC), where they kill target cells coated with antibodies.
自然杀伤（NK）细胞毒性
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