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MHC I stands for Major Histocompatibility Complex class I. These are molecules present on the surface of almost all nucleated cells in the body. Their primary role is to display peptide fragments from proteins within the cell.
When a cell is healthy, it presents normal peptides on its MHC I molecules which signals to the immune system that everything is fine.
In contrast, cells infected with viruses or transformed by tumors often present abnormal or reduced levels of MHC I.
This difference is critical for immune cells like Natural Killer (NK) cells to discern between healthy and compromised cells. NK cells recognize and spare cells with normal MHC I, while targeting and killing cells that have downregulated or altered MHC I.

The innate immune system is the body's first line of defense against infections. It includes various cells and mechanisms that respond quickly to pathogens in a non-specific manner.
Unlike the adaptive immune system, which targets specific pathogens, the innate immune system provides a rapid response. Natural Killer (NK) cells are essential components of this system.
They can recognize and kill cells without prior exposure to the pathogen.
NK cells identify infected or tumor cells by changes in their cell surface markers, particularly the absence or alteration of MHC I molecules.

• This ability allows NK cells to react swiftly, providing crucial protection before the adaptive immune system is fully activated.
• Furthermore, the innate immune system works like a barrier, preventing the spread of infections and potentially harmful cells within the body.

Pathogen recognition is a crucial function of the immune system, determining how effectively it responds to infections. Different immune cells recognize pathogens and infected cells through specific receptors.
Natural Killer (NK) cells play a significant role in recognizing compromised cells.
They do this through a balance of activating and inhibitory signals. Healthy cells present MHC I molecules that bind to inhibitory receptors on NK cells, sending signals that prevent the NK cell from killing the healthy cell.
Infected or transformed cells often lose these MHC I molecules or display abnormal ones, failing to send the inhibitory signal.

• This lack of inhibition allows the NK cell to become active and start the process to kill the infected cell.
• Effective pathogen recognition by NK cells is vital in controlling infections until a more specific adaptive immune response can be mounted.