system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is the HeLa cell line immortal?
gpt
First, let me tell you what it means when a cell is *not* immortal.

Normal human cells can divide.  1 cell becomes 2.  2 cells become 4.  4 cells become 8.  But most human cells have a limit - they can't just keep on doubling forever.  They can only divide so many times before a built-in safeguard says "that's enough, no more division."  So 8 becomes 16, 16 becomes 32, 32 becomes 64, but then it stops.  64 is the end.

An immortal cell is one that doesn't have a built-in limit.  It can keep on dividing forever: 64, 128, 256, 512, ..., one million, two million... one billion, two billion, four billion, until it takes over everything.

A cell that's abnormal but *not* immortal can form a small tumor, but the tumor's growth stops when the cells hit the division limit.  Because of that, a cell that's abnormal but not immortal isn't considered "cancer" - the limit prevents it from growing and spreading.  Instead, it's called a "precancerous condition" - the cells are abnormal and are forming a tumor, but until a mutation causes the division limit to be removed, they won't kill you - yet.

HeLa is actually a cervical cancer cell.  Like all cancer cells, it's immortal - if it weren't immortal, they wouldn't call it "cancer," they would call it "precancerous."

It is possible to take a normal human cell and remove the division limit in the laboratory - ie, "immortalize" it.  This can be useful for scientists, since they need to be able to grow the cells in the lab, and they don't want some built-in division limit to tell them they have to start over.