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Why do cells kill themselves?

As you grew in your mother's womb, apoptosis was vital for your early development. For example your feet were webbed to begin with and rows of cells died between your toes to separate them. When your brain was developing, more than half of its early nerve cells sacrificed themselves. As adults, apoptosis helps keep all our organs the right size - as new cells grow, old ones die. It is also crucial for a healthy immune system: apoptosis weeds out ineffective white blood cells.