Functions of natural killer (NK) cells NK cells in innate immunity are best described as participating in which of the following processes?

Introduction / Context:
Natural killer (NK) cells are cytotoxic lymphocytes of the innate immune system. They provide rapid responses against virally infected cells and surveillance against transformed (tumour) cells. Understanding their roles clarifies early host defense and interactions with adaptive immunity during transplantation and infection.

Given Data / Assumptions:

• NK cells kill targets lacking or downregulating MHC class I molecules (the “missing self” concept).
• They mediate antibody-dependent cellular cytotoxicity via FcγRIII (CD16).
• Cytotoxicity uses perforin/granzyme pathways and death receptors.

Concept / Approach:

In tumour rejection, many cancers reduce MHC I to evade CD8 T cells, making them NK targets. In viral infection, cells often alter MHC I or express stress ligands recognized by NK activating receptors, driving non-specific killing of virus-transformed cells. In transplantation, innate mechanisms including NK cells can contribute to allograft injury, especially when MHC disparities reduce inhibitory signaling; NK cells also modulate adaptive responses via cytokines (IFN-γ, TNF) and crosstalk with dendritic cells.

Step-by-Step Solution:

Verification / Alternative check:

Experimental models show NK-mediated lysis of MHC I–deficient targets; clinical observations link NK activity with viral control and with roles in graft outcomes, supporting their involvement across scenarios listed.

Why Other Options Are Wrong:

• Choosing only one role underestimates NK versatility.
• “None of these” conflicts with well-established NK functions.

Common Pitfalls:

• Assuming allograft rejection is exclusively T cell–mediated; innate components, including NK cells, contribute.
• Confusing NK cells with NKT cells or cytotoxic T lymphocytes.

Final Answer:

All of these