Inner worlds, outer worlds, separated by membranes. What do T cells do during an infection? Imagine, as a human immune system might see it, that there are two pathological worlds of microbes. There is an “outer” world of a bacterium or a virus floating outside the cell, in lymph fluid or blood, or in tissues. And there is an “inner” world of a virus that is embedded and living within a cell. It is the latter world that presents a metaphysical, or rather a physical, problem. A cell, we said before, is a bounded, autonomous entity with a membrane that seals it from the outside. Its inside—the
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