Natural killer (NK) cells are one of those Step 1 topics that feels “small” until it shows up everywhere: viral infections, tumor surveillance, antibody-dependent killing, cytokines, and a handful of board-favorite immunodeficiencies. If you can confidently answer what NK cells kill, how they decide, and what happens when they fail, you’ll pick up easy points across immunology, heme/onc, and infectious disease.
Where NK Cells Fit: Innate Immune Lymphocytes With Adaptive-Like Tricks
NK cells are large granular lymphocytes that belong to the innate immune system, but they share some features with adaptive immunity (they’re lymphocytes and can mediate targeted killing).
Core roles (high yield):
- Early defense against viruses (especially herpesviruses) and intracellular pathogens
- Tumor immunosurveillance
- Killing without prior sensitization (no TCR rearrangement; no MHC restriction like CD8 T cells)
- Antibody-dependent cellular cytotoxicity (ADCC) via CD16 (Fc receptor)
First Aid cross-reference (Immunology):
- NK cells: “Kill cells with low MHC I; ADCC via CD16; secrete IFN-γ.”
- Cytokines: IL-12 → stimulates NK cells; IFN-γ → activates macrophages
Definition and Key Markers (Boards Love These)
What defines an NK cell?
An NK cell is an innate lymphocyte specialized for cytotoxic killing of abnormal host cells, especially when those cells:
- Downregulate MHC I (common in viral infection and cancer), or
- Are opsonized with IgG (ADCC)
Surface markers to know
| Feature | NK Cell | Why it matters |
|---|---|---|
| CD16 | Yes | Binds Fc portion of IgG → ADCC |
| CD56 | Yes | Classic NK marker (esp. in lab/flow) |
| TCR/CD3 | No | Distinguishes from T cells |
| MHC restriction | No | Unlike CD8 T cells |
High-yield association: NK cells are commonly described as CD16+ CD56+.
NK Cell “Decision-Making”: Missing-Self Recognition
The central concept
NK cells constantly balance:
- Inhibitory signals (usually from self MHC I)
vs - Activating signals (stress ligands on infected/tumor cells)
If MHC I is missing or decreased, inhibitory signaling drops → NK cell becomes more likely to kill. This is the classic “missing-self” concept.
Why viruses and tumors are implicated
- Many viruses (and tumor cells) downregulate MHC I to escape CD8+ cytotoxic T cells
- That strategy backfires because it makes them more visible to NK cells
Step 1 phrasing to recognize:
- “Decreased MHC I expression” → think NK cell activation
- “Increased susceptibility to HSV/EBV” → think NK dysfunction (among other causes)
Mechanisms of Killing: How NK Cells Actually Do the Damage
NK cells kill by two classic pathways:
1) Perforin–Granzyme pathway (direct cytotoxicity)
- Perforin creates pores in the target cell membrane
- Granzymes enter and trigger apoptosis (via caspases)
This is the same effector mechanism used by CD8+ T cells, but NK cells don’t need antigen presentation on MHC I.
2) ADCC (Antibody-Dependent Cellular Cytotoxicity)
- NK cell CD16 binds the Fc region of IgG
- Target cell is coated with IgG (e.g., antibody against tumor antigen or viral protein)
- NK cell releases cytotoxic granules → kills the opsonized cell
High-yield differentiator:
- Complement kills via MAC (C5b-9)
- NK cells kill via ADCC when antibodies are already bound
NK Cytokines: What They Secrete and What Activates Them
What activates NK cells?
- IL-12 (especially from macrophages and dendritic cells) is a major NK activator
What NK cells secrete
- IFN-γ (the big one to remember)
- Activates macrophages
- Promotes Th1 responses
- Helps control intracellular pathogens
Board pattern:
Macrophage makes IL-12 → activates NK → NK makes IFN-γ → activates macrophage (a reinforcing loop).
Pathophysiology: What Happens When NK Function Is Impaired?
Defective NK function = impaired early control of:
- Viruses (classically herpesviruses)
- Intracellular pathogens (less “classic” than herpes, but conceptually tied to IFN-γ/macrophage activation)
What infections show up?
High yield viral associations:
- HSV
- EBV
- CMV
- VZV
- HPV (can be seen in combined immune defects where NK function is part of the issue)
Clinical pattern
- Severe, recurrent, or disseminated herpesvirus infections
- Sometimes viral-triggered HLH-like pictures (especially when cytotoxic pathways are disrupted—see below)
Clinical Presentations You Should Recognize
1) Isolated/primary NK cell deficiency (conceptual)
You may see vignettes like:
- Child/adolescent with recurrent severe HSV/CMV/VZV
- Otherwise relatively normal B and T cell counts (depending on the exact disorder)
2) Chediak–Higashi syndrome (classic cytotoxic granule problem)
While not purely “NK-only,” it’s extremely high yield because it directly hits the granule-mediated killing pathway used by NK cells and CD8 T cells.
Pathophysiology:
- Defective lysosomal trafficking (LYST mutation)
→ impaired phagolysosome formation and granule function
→ decreased NK (and CTL) cytotoxicity
Clinical clues:
- Recurrent pyogenic infections (often Staph/Strep)
- Partial albinism, peripheral neuropathy
- Giant granules in granulocytes/platelets
Why it’s relevant for NK: impaired degranulation/cytotoxic killing.
3) Hemophagocytic lymphohistiocytosis (HLH) (tie-in to NK cytotoxicity)
HLH can be triggered by infections (notably EBV) and is associated with impaired cytotoxic function of NK/CTLs in some genetic forms.
Clues (Step-style):
- Persistent fever, hepatosplenomegaly
- Cytopenias, very high ferritin
- Hypertriglyceridemia
(You’re often not asked to diagnose HLH from scratch on Step 1, but NK/CTL cytotoxic dysfunction is a recurring mechanistic explanation.)
Diagnosis: How NK Problems Show Up on Exams (and in Workups)
Step-relevant “diagnostic” handles
You won’t usually be ordering a full NK functional assay on Step, but you should recognize:
1) Flow cytometry markers
- NK cells: CD16+ CD56+
- Not CD3 (T-cell marker)
2) Functional testing (conceptual)
- Decreased NK cytotoxicity (e.g., reduced degranulation or killing assays)
3) Pattern recognition via infections
- Severe herpesvirus infections disproportionate to other findings → think NK function issues (or broader cellular immunity defects)
Treatment: What You Actually Do With NK-Related Problems
Treatment depends on whether you’re dealing with:
- Isolated NK dysfunction
- A broader immunodeficiency
- A granule/cytotoxicity disorder (e.g., Chediak–Higashi)
- HLH-like syndromes
General principles (board-friendly)
- Aggressive antiviral therapy for herpesviruses (e.g., acyclovir for HSV/VZV; ganciclovir for CMV—mechanisms are fair game elsewhere)
- Treat underlying immunodeficiency
- Hematopoietic stem cell transplant (HSCT) may be curative in severe congenital cytotoxicity disorders (commonly tested conceptually)
- Supportive care + infection prophylaxis as appropriate
Chediak–Higashi (high yield)
- HSCT can address the immune defect
- Ongoing management of infections; some patients develop an “accelerated phase” (lymphoproliferation/HLH-like), which is serious
High-Yield Associations & Exam Triggers (Rapid Review)
NK cell one-liners
- Innate lymphocyte that kills virus-infected and tumor cells
- Activated by IL-12
- Secretes IFN-γ → activates macrophages
- Kills via perforin/granzymes
- Performs ADCC via CD16
- Targets cells with decreased MHC I (“missing self”)
Fast table: NK vs CD8 T cells (common confusion)
| Feature | NK cell | CD8+ T cell |
|---|---|---|
| Requires antigen presentation | No | Yes (on MHC I) |
| Key trigger | Low MHC I, stress ligands, IgG-coated targets | Specific peptide–MHC I |
| Cytotoxic mechanism | Perforin/granzymes | Perforin/granzymes |
| ADCC | Yes (CD16) | Not the classic role |
| Adaptive memory | Not classic (innate) | Yes |
“If you see this, think NK cells”
- Tumor cell downregulates MHC I → NK cells compensate
- Recurrent severe herpesvirus infections with otherwise unclear immune defect
- CD16+ CD56+ on flow cytometry
- “ADCC” in the stem → NK cell is the answer most of the time
First Aid Cross-References (What to Re-skim)
While page numbers vary by edition, these are the exact First Aid concepts to cross-check:
- Innate immunity: NK cells as large granular lymphocytes
- Cytokines: IL-12 activates NK; IFN-γ activates macrophages
- Cell-mediated immunity: perforin/granzyme killing
- Immunodeficiencies: Chediak–Higashi (lysosomal trafficking defect; impaired NK/CTL killing)
Mini Self-Quiz (Board-Style)
-
A tumor cell line is engineered to downregulate MHC I. Which immune cell becomes more important for killing it?
Answer: NK cell -
An immune cell binds the Fc portion of IgG using CD16 and induces apoptosis of the target. What is this mechanism called?
Answer: ADCC (antibody-dependent cellular cytotoxicity) -
Macrophages release a cytokine that activates NK cells early in infection. Name the cytokine.
Answer: IL-12 -
NK cells secrete a cytokine that activates macrophages and promotes Th1-type responses. Name it.
Answer: IFN-γ