The passage of red blood cells through the walls of capillaries without rupturing them, allowing these cells to move from blood vessels into surrounding tissues.
From Greek 'dia-' (through) + 'pedesis' (a leaping), derived from 'pedân' (to leap or spring). The term emerged in medical literature in the 19th century to describe the mysterious ability of blood cells to pass through intact vessel walls.
This process is essential for your immune system to work—white blood cells use diapedesis to escape bloodstream 'highways' and patrol tissues for infections, yet for over a century doctors couldn't figure out exactly *how* cells squeezed through seemingly solid walls without tearing them.
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