One fold. Many different jobs. Antibodies, T-cell receptors, MHC molecules, cell adhesion proteins, even a giant muscle protein like titin — all of them use the same ancient structural building block: a small β-sandwich domain called the immunoglobulin fold. It first showed up in evolution long before immune systems as we know them existed. Then, over hundreds of millions of years, it got duplicated, again and again, and each copy drifted off to do something completely different: → In antibodies, it became a shape-shifting recognition system that can bind virtually any molecule on Earth → In MHC proteins, it became the machinery that shows the immune system what's happening inside your cells → In NCAM and other adhesion molecules, it became the glue that lets neurons find each other and build the brain → In titin, it became a mechanical spring inside your muscles Same fold. Wildly different destinies. It's a striking reminder that evolution copies what already works, then lets each copy wander in a new direction. The immunoglobulin superfamily is one of the clearest examples of this principle in the entire genome, and it's a big part of why your immune system, your nervous system, and even your muscles all quietly speak a shared molecular dialect. What's your favorite example of one biological design being reused across wildly different systems?