Homeostasis and immunity

For a first blog post I decided to talk about homeostasis, which is a dynamic regulation of a system's internal environment, and the immune system. The usual choices for this topic when talking about humans would be the nervous and endocrine systems, but I wanted to explore what the immune system has to do with maintaining homeostasis. Later posts will examine this relationship more closely, so as to present the topic with an incrementally increasing level of detail.

To begin with, briefly on homeostasis. The term encompasses a huge portion of nature and all of life. It can be thought of as what gives a system the ability to keep itself comfortable despite pressure from the environment.
Wherever we turn our attention, some sense of homeostasis always pops up, especially if you are trying to find it. It is present in cells as they use pumps to maintain a certain ion concentration, it is evident in an organism which protects itself from overheating by evaporating water from the skin, it is noticeable in society as countries try to maintain their independence and survival by diplomatically regulating both domestic and foreign affairs.

No matter where one looks, homeostasis can always be identified by locating the three main components: sensor, integrator and effector. 

First of all, there must always be something that disturbs the system, like a virus which wreaks havoc or has the potential to do so, or an elevation/fall of some vital parameter, like temperature.

The sensor notices that something is wrong and begins the homeostatic process by informing the integrator about the impending doom to the system's stability and ability to function. The integrator decides what is to be done and transmits commands to the effector which reacts accordingly to the stimulus and if everything goes right restores the normal status of a system.

Of course, even though this description implies that homeostatic mechanisms come into action only when a disturbance occurs, actually due to there being a whole lot of different parameters to be maintained, some mechanisms are always in action. For example the mentioned pumps work constantly by making sure that the intracellular fluid contains more potassium and is more negative than the extracelullar fluid which should be more positive and rich in sodium.

Now, the fine line between a sensor, integrator and effector is pretty clear when talking about the nervous and endocrine systems. Dominika Dabrowski has written a nice overview of thermoregulation: when the temperature drops "this stimulates skin cold receptors (increase in their activity) and cools the blood flowing into the skin. These signals are received by both the hypothalamic thermostat and higher cortical centers. The thermostat is also activated by the change in blood temperature. It initiates responses that promote heat gain and inhibits centers that promote heat loss".

We can immediately identify that the skin receptors are the sensors, the hypothalamus and cortical centers are the integrators. What's left are the effectors which respond to the hypothalamus by inducing involuntary muscle contractions (shivering) to produce heat, vasoconstriction to reduce heat loss, etc.

However, in the immune system, things get quite different and actually the mechanism is not so straightforward as mentioned in the general example. Pathogens are not like temperature. Temperature is just a certain value of a property, while various germs are unwelcome guests and their existence is an absolute value: you can either be free of germs, or infected with germs. There is no middle ground.

Therefore, it is not so unexpected to see some variability in the mechanism of homeostasis, even though the goal remains internal stability. There may be some generalized effectors which remove detrimental stimuli without them being sensed previously, and only after this general effector is breached, the sensor picks up the signal and starts a real, initiated response. The sensors could also be the effectors at the same time. Dendritic cells, for example, swallow the pathogens (effector action) and then present the pathogen on their surfaces so that it can be recognized (sensor action).


Talking about generalized effectors, it is obviously beneficial to be able to ward off unwanted micro-guests without expending any energy, or as little as possible. And since the general consensus the body makes is that "either you're me or you're against me" (the failure of following this consensus will be the topic of a later post), the body has barriers - generalized effectors - which block easy entrance of pathogens into the body. The skin envelops the body and wards of weaponless microorganisms, but they can still get inside the body via openings, such as mouth, nose, ears and gain access to sensitive internal environment of the respiratory or gastrointestinal system. However, the body won't be fooled so easily. The mucous membranes which are present in these areas produce mucous secretions, for example nasal mucus in the nasal passages. Mucus traps small particles such as dust, pollen, also bacteria and viruses which in the absence of mucus would be able to easily penetrate into the lower respiratory tract and other sensitive areas. So when you unpleasantly notice that your mucus is greenish or yellowish, realize that this is the first line of defense doing its job and trapping the bacteria or viruses within itself.

Unfortunately, this superficial defense is very limited and not so effective in maintaining homeostasis against any serious foe. That is obvious, since each year millions of people get colds and other sorts of viral, bacterial and fungal infections. Germs, like Rhinovirus, which remain on the periphery of the respiratory and gastrointestinal tracts cause mild infections like the common cold. Others, mainly bacteria, can induce moderate ones: pharyngitis, laryngitis, etc.
Pathogens can also penetrate the skin, for example, with the help of our misfortune of getting a splinter or a bruise. They can travel in company with ectoparasites, like Plasmodium falciparum does with Anopheles mosquitos and when the mosquito tries to get blood it inadvertently allows Plasmodium to enter the bloodstream and causes one of the most detrimental infectious diseases - malaria.

Fortunately, the immune system is not composed solely of the skin and some slimy mucus. Such external barriers represent only one part of the innate immunity, which together with the acquired immunity represent the way that our organism responds to pathogenic stimuli which threaten its homeostatic stability and sometimes its very existence.

So the immune system is directly linked with maintaining homeostasis both constantly, i.e. with a constant presence of barriers to pathogens, and also upon being induced, when those pathogens breach the general defenses and induce sensors to call for reinforcements.