How do pyrogens trigger fever at the hypothalamic level?

• A. Pyrogens directly raise body temperature by acting on the hypothalamus to increase heat production order without cytokines.
• B. Pyrogens stimulate hypothalamic neurons to shutdown; decreasing set point.
• C. Pyrogens stimulate immune cells to release cytokines, which induce COX activity and increase PGE2 synthesis in the hypothalamus, raising the set point.
• D. Pyrogens increase skin blood flow to dissipate heat rapidly.

Answer: (C)

Fever happens because pyrogens set off a cascade that raises the hypothalamic thermostat. Pyrogens, from bacteria or other sources, stimulate immune cells to release cytokines such as IL-1, IL-6, and TNF-α. These cytokines boost COX activity in the hypothalamus, leading to more prostaglandin E2 (PGE2) production. PGE2 then acts on hypothalamic receptors to raise the body's temperature set point. In response, the body turns on heat-producing and heat-conserving mechanisms—shivering and vasoconstriction—to reach that new, higher set point, which is experienced as fever.

This explains why the process relies on cytokines and PGE2 rather than a direct action by pyrogens on the hypothalamus, and why the fever involves increasing the set point rather than lowering it. It also clarifies why increasing skin blood flow would not generate fever—it would help dissipate heat instead of elevating the set point.