Natural killer (NK) cells are particularly effective against malignant tumors that have:

• A. Increased expression of class I MHC molecules
• B. Decreased expression of class I MHC molecules
• C. Expression of viral antigens
• D. High levels of oncofetal antigens

Answer: (B)

Natural killer (NK) cells play a crucial role in the immune system's response to cancer and virally infected cells. They are unique among lymphocytes because they do not rely on antigen recognition through traditional receptors like T cells. Instead, NK cells identify stressed cells in the absence of antibodies and MHC, allowing them to react swiftly to potential threats.

Malignant tumors often downregulate the expression of class I major histocompatibility complex (MHC) molecules. This occurs as a mechanism of evasion from cytotoxic T cell responses, which rely on the recognition of tumor antigens presented by class I MHC. However, NK cells are equipped with a range of activating receptors that can detect the absence of these MHC class I molecules.

When malignant cells exhibit decreased expression of class I MHC molecules, they effectively become targets for NK cells because these immunological sentinels are activated by the lack of MHC presentation. The loss of class I MHC provides a "missing self" signal, prompting NK cells to induce apoptosis in the tumor cells.

In summary, malignant tumors that have decreased expression of class I MHC molecules are particularly susceptible to NK cell activity, allowing the immune system to detect and eliminate them despite their efforts to