Final answer:
Natural Killer (NK) cells directly use MHC class I molecules in their defense strategy of the innate immune system, where they detect and destroy compromised cells that lack this molecule.
Step-by-step explanation:
The innate immune system component that uses MHC class I molecules directly in its defense strategy is Natural Killer (NK) cells. These NK cells are a type of lymphocyte that plays an essential role in the body's first line of defense against virally infected cells and various types of tumors. NK cells recognize the MHC I receptor on healthy cells, and if MHC I is absent—often the case in infected or malignantly transformed cells—the NK cell will proceed to lyse (destroy) the compromised cell.
It's also important to note that macrophages and neutrophils are both phagocytic cells that engulf pathogens, and B cells are part of the adaptive immune system and do not use MHC class I molecules directly.