![]() Imagine if you went from the top of your ears straight into the middle of your head-the center is where the hypothalamus would be. The hypothalamus is a small command center deep in your brain. Prostoglandins can get into the brain and talk to your neurons, and they head (pun intended!) straight for the hypothalamus. The blood-brain barrier keeps most substances in your bloodstream out of your brain (which is good, because it protects your neurons from drugs, poisons, infections, and all sorts of damaging things). Prostaglandins can be released from almost any cell in the body (and that is 37.2 trillion cells per human body, in case you’re counting!) and they are small enough and special enough to cross the blood-brain barrier. One of the things cytokines do is trigger the release of prostaglandins, which are a special kind of hormones involved in illness and injury response. Cytokines are tiny proteins that have a bunch of functions in your body, and are important for the immune response and cell-cell communication. The leukocytes (white blood cells) of your immune system are releasing proinflammatory cytokines. Your immune system and brain are talking to each other. What’s actually happening is a little more complicated. That’s partly true as your immune system might use a little extra energy, but not enough to account for what we know as sickness behavior. It used to be thought that you feel tired when you are sick because all of your energy was going to your immune system. So, what about the behavioral part where you're feeling icky? Why don’t you want to talk to anyone? The immune system creates all those symptoms in an effort to fight a current infection and also defend harder against getting infected by something else while you’re already down. Coughing is triggered by dripping mucus and swelling in the bronchioles deep inside the lungs, helping you trap and expel excess mucus and invaders. Your immune system hits the histamine switch in your brain, which kicks up the activity of your mucus glands and swells up your nose to send more fluid and blood cells to that area for defensive purposes. Maybe you’ve seen Osmosis Jones and have had a primer? (If not, we recommend this as a fun at-home learning activity for the family!) White blood cells are rushing around your body, constantly patrolling and deploying specialized teams to fight off various attacks. Your immune system is charged with the job of defending you from invaders. So why do all these viruses make you feel the same way? How can hundreds of organisms all know how to stuff up my nose and make me tired and unmotivated? There are other viruses we know about, like RSV and parainfluenza, and there are a bunch that we don’t know yet. There are more than 30 kinds of coronavirus, but only a handful of them affect humans (with a range of severity). ![]() There are more than 200 kinds of rhinoviruses that cause up to 40% of our colds, and aren’t usually too bad. ![]() Almost every time you catch a cold, you’ve been infected with a new virus. ![]() There are hundreds of viruses that cause what we know as the common cold.
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