Body temperature is controlled by a part of the brain called the hypothalamus. The hypothalamus is sometimes called the body's thermostat, and it is normally set at 37. It is believed that certain substances affect the hypothalamus by setting it at a higher temperature. Recall from Chapter 15 that when phagocytes ingest gram-negative bacteria, the lipopolysaccharides of the cell wall are released, causing the phagocytes to release the cytokines interleukin-I, along with TNF-a. These cytokines cause the hypothalamus to release prostaglandins that reset the hypothalamic thermos tat at a higher temperature, thereby causing fever.