In this section we will make a small detour from our general discussion on the topic of stress and the suppression of the immune system. Here, we will examine one particular aspect of the immune system with which we are all so familiar that we tend not to give it any thought—I am talking about the symptoms of feeling sick. Typically, when we get an infection, we have a fever, our joints and muscles hurt, we feel sleepy, lose our appetite and are generally lethargic.
These symptoms are so common that usually, we use them to identify when we are sick. The symptoms occur with any other symptoms that may be particular to the specific infection but this basic set does not change. One wonders why different infectious organisms lead to so many common symptoms in humans? The surprising answer is that it is not the infectious organism causing the symptoms but our body that is producing them! For the rest of the story read on…
In an earlier section, we saw how the macrophages identify an infectious agent and raise the metaphorical alarm using the messengers interleukin-1 and -2. It turns out that the messengers are no ordinary carriers but have a surprising array of effects.
The Influential Messengers
The first part of this story starts in the brain and specifically the hippocampus, which is involved in temperature regulation. In common terms, the hippocampus is the thermostat of the body. Normally, it is set to 98.6°F It turns out that the hippocampus has a large number of interleukin receptors. What this means is that the hippocampus can sense the levels of interleukin in the blood.
The interleukin binds to the receptors in the hippocampus and causes the temperature to be set higher. We begin to feel cold, and muscles start shivering to generate heat. Blood is diverted from the periphery to the internal organs and you start feeling cold— time to pile on the blankets and bring out the sweater. Just to give an idea of the energy involved in this process, in case of malarial fever, the body consumes almost 50 per cent more energy than usual.
There are nerve pathways coming from all over the body to the spinal cord (and then to the brain) that carry pain signals. These pathways have a certain threshold that has to be crossed before the signal is accepted as painful by the brain. A simple congratulatory pat on the back does not cross this threshold and is not painful whereas a sharp blow to the back will definitely be registered as painful. Interleukin-1 does something quite insidious. It lowers the threshold for the pain signal. Suddenly, all the old aches and pains seem to come back. What the body was ignoring is now sending these pain signals and we have aching joints and muscles.
Interleukin also causes the release of a hormone—cor-ticotropin releasing factor (CRF). This is the hormone that initiates the body’s stress response and causes the release of other stress-response hormones. Soon, the symptoms associated with stress start to appear. Loss of appetite, lack of interest in sex and energy storage is blocked. If the infection continues for a long time, sperm production declines in males and females may have irregular periods or may stop ovulating altogether.
That is an impressive list of effects for a mere messenger. It seems that these are not mere side effects of some action but the body has evolved these responses and it works hard to bring about these effects. From an evolutionary point of view, it makes sense for the body to bring about symptoms like muscle pain.
A sick animal is not fit to go out in the wild looking for food—it is likely to be eaten in no time. Painful joints and muscles and lack of appetite will help in making it rest. This is only a sketchy explanation and does not cover some obvious exceptions like lions and other carnivores without any known predators. Yet, these carnivores show the same response to fighting infections as humans or other animals.