Why T cells need MHC molecules to recognize pathogens

T cells usually cannot see a germ directly

T cells usually recognize tiny pieces of a germ only after the body's own cells display those pieces on their surface using molecules called MHC. A T cell therefore does not simply see the whole germ. It sees a small sample shown to it by another cell.

Complex animals existed before the Cambrian explosion

Large and varied organisms lived before the Cambrian explosion, and some fossils preserve tracks showing that animals could move. The Cambrian explosion therefore looks less like the sudden beginning of complex animal life and more like a later period when existing animal groups spread and changed especially quickly.

Simple survivors made recovery after a mass extinction look faster than it was

After the Permian–Triassic mass extinction, a few tough kinds of life spread quickly. This made recovery look fast, but complex ecosystems took much longer to return. Research places the start of broader recovery around four to six million years later, with some estimates putting full recovery as late as 30 million years afterward.

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Why T cells need MHC molecules to recognize pathogens

T cells usually cannot see a germ directly

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