Step 1 topics → Immunology → NK Cells and Missing-Self

NK Cells and Missing-Self

USMLE Step 1 trap: Inverts the missing-self logic: MHC I inhibits NK killing rather than triggering it. NK cells are inhibited by MHC I via inhibitory receptors (KIRs); loss of MHC I removes this inhibition, triggering NK killing of the target cell.

NK cells are innate lymphocytes that kill without prior sensitization — no antigen presentation required. The core concept USMLE Step 1 tests is the 'missing-self' model: NK cells are normally held in check by inhibitory receptors (KIRs) that recognize MHC class I on healthy cells. When a cell downregulates MHC I — as many viruses and tumors do to evade CD8 T cells — that inhibitory brake is removed, and the NK cell kills. The exam also tests NK cells' antibody-dependent role via CD16 (FcγRIII), and the elegant complementarity between NK and CD8 T cell logic.

The exam hits this concept from multiple angles: pure mechanism recall (what happens when MHC I is lost), clinical application (why patients with NK deficiency are susceptible to herpesvirus infections), and passage interpretation (a scenario where a cell lacks MHC I and you need to predict which effector will kill it). Expect vignettes that force you to distinguish between CD8 T cell activation versus NK activation based on MHC I expression status.

The most common trap on USMLE Step 1 is inverting the missing-self logic — students often think MHC I is a kill signal for NK cells rather than an inhibitory signal. The second trap is confusing CD3 (a T cell marker) with CD16 as the ADCC receptor on NK cells. Get those two right and most NK cell questions become straightforward.

From real student decks
62%have cards covering this topic
27%have mature cards

Common misconceptions

Common mistake
Wrong: NK cells kill cells that express MHC I because MHC I is a kill signal.
Right: NK cells are inhibited by MHC I via inhibitory receptors (KIRs); loss of MHC I removes this inhibition, triggering NK killing of the target cell.
MHC class I does the opposite of what most students assume — it is an inhibitory signal, not a kill signal. Inhibitory KIRs on NK cells bind MHC I and tell the NK cell 'this is a normal self cell, stand down.' When a virus or tumor downregulates MHC I to hide from CD8 T cells, it inadvertently removes that inhibitory brake, and the NK cell activates and kills. Reframe it this way: MHC I = inhibition of NK killing; no MHC I = disinhibition = kill.
Common mistake
Wrong: CD8 T cells and NK cells use the same mechanism to identify targets.
Right: CD8 T cells kill cells that display antigen on MHC I (requires MHC I), while NK cells kill cells that have lost MHC I — the two mechanisms are complementary and cover each other's blind spots.
CD8 T cells and NK cells actually use opposite logics to find their targets. CD8 T cells require MHC I — a cell must display antigen on MHC I for a CD8 T cell to recognize and kill it. NK cells are activated precisely when MHC I is absent. This isn't redundancy; it's a complementary system where viruses and tumors can't win by simply turning MHC I off — doing so just hands them over to NK cells instead of CD8 T cells.
Common mistake
Wrong: NK cells use CD3 to mediate antibody-dependent cellular cytotoxicity.
Right: NK cells express CD16 (FcγRIII), which binds the Fc region of IgG coating a target cell, triggering ADCC; NK cells do not express CD3.
NK cells do not express CD3 — that's a T cell receptor complex marker. For ADCC, NK cells use CD16 (also called FcγRIII), which is an Fc receptor that binds the constant (Fc) region of IgG antibodies already stuck to a target cell. Once CD16 crosslinks those antibody-coated targets, the NK cell releases perforin and granzymes to kill. Remembering 'CD16 on NK' versus 'CD3 on T cells' is a classic Step 1 marker distinction.
Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →
Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

  1. Understand the missing-self mechanism: MHC class I normally inhibits NK cells via inhibitory receptors (KIRs), so loss of MHC I removes that inhibition and triggers NK killing — not the other way around.
  2. Know how CD16 (FcγRIII) mediates ADCC: NK cells bind the Fc region of IgG antibodies already coating a target cell through CD16, then release perforin and granzymes to kill — this is separate from the missing-self pathway.
  3. Recognize the complementary logic of CD8 T cells and NK cells: CD8 T cells require MHC I to kill (they kill cells displaying antigen on MHC I), while NK cells are activated by the absence of MHC I — together they close each other's blind spots against viral and tumor immune evasion.

Can you avoid these mistakes?

A tumor cell mutates to stop expressing MHC class I molecules. Which effector cell becomes MORE likely to kill it, and why? Which effector cell becomes LESS effective against it?
An NK cell encounters a target cell coated with IgG antibodies. The target cell has normal MHC I expression. Will the NK cell kill it, and through what receptor and mechanism?
A patient presents with recurrent severe herpesvirus infections since childhood. Lab work shows absent NK cell activity but normal T and B cell counts. Which step in NK cell killing is most likely defective — the inhibitory receptor pathway, ADCC via CD16, or the perforin/granzyme release pathway — if the defect specifically prevents recognition of antibody-coated targets?
True or False: A cell that downregulates MHC I to evade CD8 T cells is now safe from all lymphocyte-mediated killing. Explain your reasoning.

Related topics

Thymus and T Cell Maturation Spleen Architecture and Function Lymph Node and MALT Architecture Innate Immune Cells Overview

See how your Anki deck covers this topic.

Upload your deck for a free audit →