Why your immune system makes you feel rotten

The misery of being sick. You’re lying inert on the couch, and it´s not so much the cough, or the aches and pains, but just an overall feeling of despair, of the world ending, of you having become the filling in a doom sandwich.

Fecking viruses and bacteria making you feel like this.

However, as it turns out, it´s not really them that make you feel like the devil´s haemorrhoids — it is your own immune system that´s mostly responsible for that, thank you very much. Your immune system, who is supposed to be your friend and bodyguard, your first line of defence against the worst scum of the universe, is the one that causes most of the misery.

You know when a friend gives you truly good advice and you know it is in your best interest to follow it, but you really don´t want to do it? Well, your immune system doesn’t deal in advice, it simply knocks you out and embeds you into that couch, because it can actually go behind your back and deal directly with your brain. That’s right, your immune system and your brain conspire together to make you feel like you were just thrown up by a garden gnome.

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How do they manage to work together? Cells from the immune system can talk to each other using chemical compounds known as cytokines. These are little chemical messages that immune cells can send out, and any cell that has a cytokine receptor (which works like a sort of antenna) can pick these up. Usually, the messages sent by cytokines have the goal of co-ordinating a military-like response to an invader: raising the alarm or calling for reinforcements are typical examples. But many cells in the body have cytokine receptors, not just immune cells, and so cytokine messages allow for communication across many different organs and systems.

For example, cytokines acting on the temperature-controlling part of your brain can start a fever. Feeling feverish is not great, but the increased temperature helps the cells of your immune system to travel faster through your body and reach the battlefield in sufficient numbers to put up a good fight against the enemy.

But it´s not just that. Cytokines acting on your brain can also trigger what is known as sickness behaviour: a set of subjective feelings of sickness including malaise (general discomfort), lethargy, fatigue, increased sensitivity to pain and anhedonia (the inability to feel pleasure), as well as reduced social interaction. In summary, misery, end of the world, devil´s haemorrhoids.

Why would your immune system do this to you?

Isn’t it meant to be your mate?

Don’t you feed it every day with those fancy yoghurts?

Does it actually hate you?

No, your immune system doesn’t hate you, but it is quite ruthless and in order to do its job properly it will not hesitate to make you feel like you´ve been stampeded on by a pack of Ed Sheeran fans at the gates of Croke Park. Like any army, your immune system requires loads of supplies to fight, including fuels such as sugar and oxygen. If you’re gallivanting around, going out with your mates and spending your energy uselessly in drinking pints and having fun, your immune system will not be able to perform properly. If you´re feeling like a stomped-on cigarette end, though? Top of the morning for your immune system.

Messages of doom

So yes, it makes perfect sense that your immune system makes you feel like crap for a while, in order to make you save energy that it can then use to fight whatever is making you sick. But too much of a good thing is not good at all, and levels of cytokines flying around with their messages of doom go down once the enemy is defeated, and with them the sickness behaviour itself. All quiet on the Western front.

Scientists have noted the similarities between sickness behaviour and depressive disorders. A large proportion of patients with cancer or viral infections treated with cytokines such as interferon alfa develop major depressive disorders; these are also more common in people who suffer chronic inflammation than in the general population. However, sickness behaviour is reversible, and there is only a partial overlap with major depressive disorders. Some researchers have postulated that major clinical depression could be caused by an excessive (in terms of intensity and/or duration) inflammatory response, but more research is needed to clarify this.

Recent research suggests that the interaction between our brains and our immune systems goes much further than keeping you down so you don’t waste energy. Inflammation generated by the immune system might actually influence our social behaviour to reduce our vulnerability to disease and increase our chances of survival. For example, inflammation may send signals to our brain to increase our interactions with those people giving us positive social feedback. Furthermore, people who are more socially isolated may have increased inflammatory activity, putting immune cells on high alert.

So our immune system might then sneakily direct us to surround ourselves with sound people that may take care of us while we’re lying inert on the couch with barely enough energy to watch Emily in Paris; if it can´t find anyone, then it ups its game because it knows tough times are coming. Overall, a master of tough love.

Well played, immune system. Well played.