The question of why our temperature goes up during illness can be split into two parts. First, you need to know what makes the temperature go up, and secondly, what advantage an increase in temperature offers.
The increase in core temperature observed during illness is commonly called fever and occurs in response to infection by a pathogenic organism or certain types of physical injury.
For example, when a person becomes infected with bacteria, the white blood cells of the immune system recognize the incoming pathogen as foreign and initiate the first stages of the immune response – the acute phase. In this reaction, white blood cells called monocytes release a variety of proteins called cytokines. These are central to the immune and inflammatory response.
In particular, there is a predominance of two types of cytokine called interleukin-1 (IL-1) and tumour necrosis factor-alpha. These cytokines are known as pyrogenic because they cause an increase in body temperature. It is not clear how they induce fever, but it is known that they also cause the production of other chemicals in the brain. The main groups of chemicals produced in this effect are the prostaglandins. These react very strongly with the hypothalamus area of the brain, which then sends a signal to the body to increase the temperature.
The mechanisms that the brain employs to affect this are not certain but are known to include increasing the metabolic rate and including shivering. These two processes burn metabolic fuel faster than normal and body heat is given off. The question of what advantage fever confers is interesting.
Experimental work shows that the mortality of animals decreases if the fever is untreated, that elevated temperatures can enhance certain aspects of the immune response. Furthermore, the growth rates of various types of bacteria are slowed at temperatures above normal body temperature. Indeed, the ancient Greeks believed that fever was beneficial; even in this century fever has been used to treat certain illness. For example, syphilis used to be notoriously difficult to treat, so doctors gave their patients malaria which fought the syphilis knowing that they could get rid of the malaria later.
With a few exceptions, our body temperature rises in response to infection. Our immune system becomes activated and seeks to destroy the source of infection.
The macrophages produce a protein signal called IL-1, which triggers the destruction of the infection and is also responsible for elevating the body’s temperature. This has three possible functions: raised body temperatures may affect the ability of micro organisms to replicate; T cells work best at temperatures of between C and C; during fever, the level of iron circulating in the body falls and micro organisms need iron in order to replicate.
So it is to our advantage that our body temperature rises when we have an infection. Incidentally, many of the symptoms we experience when we have an infection – tiredness, fever, sleepiness, aching joints and a lack of appetite – are due to IL-1 and its related proteins, and not the micro organisms. It is our body’s ways of making us slow down so we can recover from an infection and in most cases it works.