Eli5 Why do antigen tests take 15 minutes when you can see the lines right away?

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When taking rapid antigen tests and pregnancy tests it tells you to wait 15 mins (or 5 for pregnancy) but the lines show up right away. Why wait the full 15? Do the lines actually change over a period of time?

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The control line shows up fast since it’s going against a protein that you got plenty of in your nose or throat. It needs a little amount of material collecting at that line, that’s why the line comes up right away the moment the fluid passes over it. The other one can detect smaller amounts of virus, and this takes more material to flow over the area in order to collect enough “color readout”.

Everyone (4 people) in my house tested positive for Covid recently. For 3 of us, the line showed up right away. For 1 of the adults I live with the second line showed up 10-15 minutes later. We thought it was weird she didn’t test positive and then noticed the second line just took longer to appear.