Immune cells vs. invaders: it’s a war going on in every healthy human body. When the combatants travel to space, say NASA scientists, curious things happen.

In space, these cells don’t work the way they do on the ground. T-cells, for example, don’t multiply properly; there aren’t as many of them as there should be. They can’t move well. They don’t signal each other as effectively. Overall, they seem less able to destroy invading germs.
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