What is the mechanism of biological response modifiers?

Biological response modifiers, also known as immunotherapy agents, work by enhancing the body's natural immune response to cancer. These agents can activate immune cells, increase the body's production of interferons and interleukins, and create a more favorable environment for the immune system to target and destroy cancer cells. The exact mechanisms of how these modifiers exert their effects are complex and often not fully understood, which is why the correct answer notes that they provide an anti-cancer effect that is not clearly understood. This uncertainty reflects the intricacies of immune system interactions and tumor biology.

Other options present specific attributes or limitations that are not characteristic of biological response modifiers. They are not limited to cytotoxic effects, do not necessarily require combination with other drugs to be effective, nor do they rely on cell cycle specificity to function. This highlights the unique nature of biological response modifiers as a distinct therapeutic approach in oncology.