Fever: A Nearly Ubiquitous Immune Response

by J. Bruce Neill, Ph.D.

When the human body recognizes a foreign particle (cell or object) in it, a complex series of events occur that are designed to protect the body, an immune response. The spectrum and complexity of immune responses are varied and highly complex, but a common component is raising the temperature of the immediate area, or the entire creature. When an organism’s body temperature is increased, it is called a fever; the common medical adjective for exhibiting a fever is febrile.

Fever is a hallmark of immune responses. And, it has been around for approximately 600 million years. Our best evidence suggests that around that time, there was a divergence in animal body types. Prior to that, all animals’ body temperatures were mostly controlled by the surrounding external environmental temperatures – we call these animals ectotherms. Frequently known as cold-blooded, ectotherms include amphibians, reptiles, insects, and many fish species.

600 million years ago, a new development appeared, internal body temperatures that are typically above the ambient external environment, and relatively constant – the endotherms. Examples of endotherms, commonly referred to as warm-blooded animals, are mammals, birds, and some fish (notably tuna).

Both endotherms and ectotherms utilize fever as an immune response to foreign invasions. Yes, cold-blooded animals have fevers! They mostly accomplish this through behavioral modifications – moving to, and remaining in areas that increase their internal temperature. When ectotherms are treated with human drugs designed to reduce fevers (antipyretics) – they cause the cessation of the behaviors that led them to increased body temperatures. This tells us that the nervous system pathways that increase the metabolic rate and internal temperature of warm-blooded animals, are very similar in the cold-blooded animals, but they change behavior, not metabolic rate.

Perhaps, even more surprising is that fever is also known in plants. Several investigations have documented the fever response to infections in controlled experimental conditions with a variety of plants.

The creation of fever is how our body (and seemingly many varied types of organisms) wards off infections. Fever makes us feel crummy; our system is on overdrive; we burn through a lot more calories; we are tempted to take medications to reduce the fever. Significant research demonstrates that survival is increased when fever is not suppressed by drugs. Typical immune response fevers make us feel bad, but they allow us to heal more quickly.

Please stay well, enjoy small things, value one-another and wash your hands often.

Dr. Neill is the Director of Education at Sanibel Sea School. Part of the SCCF (Sanibel-Captiva Conservation Foundation) Family, Sanibel Sea School’s mission is to improve the ocean’s future, one person at a time.

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