White blood cells are constantly tearing holes in your blood vessel walls. But these guardians of the immune system are doing it to protect you: Once they ride through the bloodstream to infected tissues—where they make antibodies and eat foreign invaders—they need a way to get inside. Now, scientists have discovered just how they do it without permanently damaging blood vessels, which they slip into and out of up to 10 times each day. First, researchers added fluorescent tags to their nuclei and to the structural fibers of blood vessel walls, which keep out foreign particles and seal in blood, plasma, and immune cells. The researchers then tracked the process with video-microscopy. They found that blood vessel cells were not the ones making the openings, as previously thought. Instead, immune cells make their own way across. By softening their bulky nuclei and pushing them to the front edge of their cells, white blood cells probe apart scaffolding in the blood vessel walls and squeeze through, researchers report online today in Cell Reports . This process (seen above) snaps smaller, threadlike fibers that form the flexible scaffolding of blood vessel walls; the cells easily repair that breakage later as part of routine cellular maintenance. The researchers hope to use their discovery to better understand how metastatic cancer cells migrate into the bloodstream and spread cancer throughout the body.