NK cells kill cells that are missing MHC I. CD8 T cells kill cells that are displaying MHC I with a foreign peptide inside it. These are inverse detection strategies: CD8 T cells respond to abnormal antigen presentation, NK cells respond to the absence of normal antigen presentation. When a tumor cell downregulates MHC I to hide from CD8 T cells, it becomes visible to NK cells.
The common mistake
On an open-ended probe, Omar correctly stated both rules: "NK cells kill cells that do NOT have MHC I. CD8 cells must bind to MHC I to yield cytotoxic effects." Then he applied them backwards — concluding that the patient with absent NK cells could still attack an MHC I-negative tumor, when the opposite is true.
A lot of students get this wrong in the same way. The rules are memorizable. The application under scenario pressure is where it breaks. The typical error is stating "CD8 T cells can still kill MHC I-negative cells" or, as in Omar's case, correctly retrieving the rules but losing track of which patient has which deficit when the scenario introduces two patients simultaneously.
The trap is that the rules feel like they should stack — if both CD8 cells and NK cells are active, won't one of them kill the tumor? The answer depends entirely on which cells are present and what the tumor is displaying.
The actual mechanism
All nucleated cells express MHC I. It serves as a "don't kill me" signal to NK cells. NK cells have inhibitory receptors (KIR receptors) that bind MHC I; while those receptors are engaged, NK cells stand down. When MHC I disappears — due to viral infection, malignant transformation, or deliberate immune evasion — the inhibitory brake is released, and NK cells kill the target cell.
CD8 T cells work the opposite way. They require a specific MHC I–peptide complex on the target cell surface. Their T cell receptor binds to it; if the peptide is foreign (viral or tumor-derived), the CD8 T cell kills the presenting cell. Without MHC I on the surface, CD8 T cells have nothing to bind — they cannot recognize the target at all.
The scenario logic: - MHC I-negative tumor, NK cells absent, CD8 cells intact: CD8 T cells can't see the tumor (no MHC I to bind). NK cells are the right weapon, but they're missing. The patient is defenseless against that tumor. - MHC I-negative tumor, NK cells intact, CD8 cells absent: CD8 T cells are irrelevant (same reason). NK cells detect "missing self" and kill. The immune response is effective.
This is a real tumor evasion strategy. Some cancers selectively downregulate MHC I to escape CD8-mediated killing. The immune system's answer to that strategy is NK cell surveillance. Tumors that shed MHC I gain one advantage (CD8 evasion) while losing another (NK cell tolerance). Understanding this trade-off is what Step 1 vignettes on tumor immunology are testing. The broader principle — that different immune mechanisms cover different threat types — also governs how complement deficiencies produce distinct bacterial susceptibility patterns depending on which step of the pathway is missing.
How to remember it
CD8 T cells are the readers — they need the MHC I text to be present before they can respond.
NK cells are the monitors — they check that MHC I is present at all. No signal = kill.
They are complementary. What one misses, the other catches.
Check yourself
A cell line derived from a lung tumor has completely stopped expressing MHC I on its surface. In a patient whose immune system is otherwise intact, which effector cell is most responsible for killing these tumor cells?
A) CD8+ cytotoxic T cells
B) CD4+ helper T cells
C) NK cells
D) Regulatory T cells
Answer: C. NK cells use the "missing self" principle — absence of MHC I removes the inhibitory signal, freeing NK cells to kill. CD8 T cells cannot recognize cells without MHC I and are ineffective against this target.
Close the gap
The tutor that worked with Omar through two consecutive open-ended probes — catching the rule-application reversal both times and building toward clean retrieval — is available for your sessions too.