Fahrenheit 100 and Rising
When you are well, your body temperature varies only a little around 37o C. (98.6o F.), whether you're sweating in a steam room or hiking in the Yukon. The hypothalamus in the brain controls body temperature. It works like a thermostat, sensing the temperature of your blood. When a pathogen (disease-causing microbe) invades, however, the body fights back with every weapon in its arsenal. Heat is one of them. The immune system sends chemical messages to the hypothalamus, signaling the need for a rise in body temperature. In response, the hypothalamus causes the pituitary to release a hormone called TSH (for thyroid stimulating hormone).
TSH travels through the blood and reaches the thyroid gland in the neck. There, it stimulates the thyroid to make another hormone, thyroxine. Thyroxine travels to all the cells of the body. It makes them burn food faster, generating more heat. The result is a fever--defined as a body temperature of 100o F. or greater. Many pathogens can't survive such high temperatures, so the fever kills them. As the pathogen begins to lose the battle, other chemical messengers travel to the hypothalamus. The hypothalamus signals the pituitary. TSH production decreases, thyroxine levels decrease, and body cells release energy more slowly. Body temperature returns to normal.
There's a lot more to a fever than a change in temperature. Researchers find high levels of immune chemicals including interleukin-6 in people with fevers. When healthy volunteers took IL-6 in a research project, they got fevers and flulike symptoms.
