Why You Feel Fuzzy-Headed When You’re Sick

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After spending a few days in bed with the flu, we often feel, well, a bit stupid, notes science writer Carl Zimmer in a fascinating new column for Discover:

“It is a common sensation, that your sickness is slowing down your brain. At first blush, though, it doesn’t make much sense. For one thing, flu viruses infect the lining of the airways, not the neurons in our brains. For another, the brain is walled off from the rest of the body by a series of microscopic defenses collectively known as the blood-brain barrier. It blocks most viruses and bacteria while allowing essential molecules like glucose to slip through. What ails the body, in other words, shouldn’t interfere with our thinking.

But over the past decade, Jonathan Kipnis, a neuroimmunologist in the University of Virginia School of Medicine’s department of neuroscience, has discovered a possible link, a modern twist on the age-old notion of the body-mind connection. His research suggests that the immune system engages the brain in an intricate dialogue that can influence our thought processes, coaxing our brains to work at their best.”

It has long been known that the membranes encasing the brain, called the meninges, are loaded with T cells and other immune cells, Zimmer notes.

“Kipnis is now investigating what exactly the T cells surrounding the brain are doing to make the brain work well. One strong possibility: They keep the rest of the immune system from inadvertently harming it.

When we learn something new, our neurons tear down old connections and build new ones. In the process they cast off lots of molecules. To the immune system, this waste may look like an infection or some other kind of trouble, resulting in inflammation and the release of harsh compounds that normally fight viruses but can also interfere with the brain and its function.

Kipnis suggests that T cells keep this process in check, differentiating between disease and ordinary stress and, when warranted, telling other immune cells to stand down by releasing antagonist molecules that prevent misguided inflammation.

The same T cells that protect the brain from inflammation also work to keep us sharp; and in what appears to be a feedback loop, the mere act of learning reinforces the effect.”

This theory could explain why we lose our mental edge when we get sick, Zimmer writes:

“When we’re healthy, T cells keep the immune cells in the meninges from inflaming the brain. But when we get sick, the T cells loosen their hold to let the immune system attack invading pathogens. The resulting inflammation helps clear out the invaders, but it also blunts learning. When we’re sick, Kipnis proposes, it’s more important to launch a powerful immune attack than to have a sharp mind. ‘Everything in life is priorities,’ he says.” (Read more here.)

I have definitely noticed this lack of mental sharpness when I’ve been sick. Interesting, and a little scary, to think about what our intellectual functioning would be like without the action of these crucial cells.

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