Here’s something you probably haven’t given much thought to: Why do tattoos last forever?
这也许是你很少会思考的问题：为什么纹身能够永久留在皮肤上呢?
The answer is more interesting than you might think, which is that your body thinks it is under attack when someone draws on it.
这个问题的答案也许比你想象的更有趣——你的身体在纹身过程中会认为它受到了袭击。
It’s actually very difficult for anything to stay on the skin, The Independent reports. We shed about 40,000 skin cells per hour, or around a million per day. Anything drawn onto the skin — by a pen, for example — gradually flakes or gets washed off.
英国《独立报》报道显示，实际上物质很难在皮肤上停留很久。每个小时我们大约会脱落4万各皮肤细胞，或者是每天一百万个左右。只要是用笔之类的东西画在皮肤上的东西，都会逐渐地脱落或者被洗掉。
So how do tattoos avoid shedding? The answer is pretty simple — the gun (or other tool used to create the art) pushes the ink through the outer layer of the skin and into the dermis, which is a deeper layer of skin that doesn’t flake off. It’s composed of collagen fibers, nerves, blood glands, and more.
所以纹身是如何避免脱落的呢?其实答案非常简单。纹身枪(或者是纹身工具)透过真皮层把颜料注入真皮层。真皮层是人体皮肤更深的一层，不会脱落，由胶原纤维、神经、腺体等其他组织构成。
Every time a needle penetrates, it causes a wound. This alerts the body to begin the inflammatory process, which sends immune system cells that try to start repairing the skin — and while this sounds like a bad thing, it’s actually what makes the tattoo last.
每回细针扎入皮肤，就会造成创伤。这就会促使身体发言，这就给免疫细胞传递了修复皮肤的信号。尽管这听上去不那么美好，这的确是纹身保持永久的秘诀。
The cells try to “eat” the ink and transfer it away from the dermis, but there is more ink than they can break down, so the artwork stays. This is also a window in how laser removal works — the laser breaks down the ink, allowing the cells, called macrophages, to transfer the ink away.
细胞们试图“吞噬”颜料，然后将其逐出真皮层。然而，真皮层中的颜料数目非常大，它们无力消化。所以纹身的图案能够永久保存下来。这也是激光去纹身的工作原理：激光破坏了颜料的结构，让这些巨噬细胞得以将颜料转运出真皮层。