Fever
A fever is an increase in body temperature to over 100°F or 37.8°C. When the body’s temperature rises, blood vessels dilate, or widen, to release heat into the air around the body. This causes the skin and mucous membranes to become red or flushed, which is why you often see flushed faces when someone has a fever.
The tongue is made up of mucous membranes, so it will appear redder than usual when a person has a fever. That redness will not be evenly distributed throughout the tongue. The tongue’s top surface contains papillae, projections containing taste buds, and sensory cells, so your tongue may have more redness on its top surface than on its bottom surface.
However, if you have a fever, you will probably notice that your tongue becomes white in patches, especially if you look at it in natural light. This is because those papillae contain pigmentation cells called melanocytes; these cells contain a pigment that helps protect the tongue from the sun’s ultraviolet rays. If your temperature rises, those pigment-containing cells will be destroyed, and the red color will fade away.
