How are tattoos chemical reactions?

Answer 1

They aren't, so far as I know, but there's a biological effect there.

There's a process in your body called phagocytosis. Basically it's garbage collection. If a foreign substance enters your body, special white blood cells called phagocytes will go up to it and bite little pieces off. They'll carry the substance to your large intestine and throw it in, where it will be added to your stool and...well, crapped out.

When you get a tattoo, the artist will go over each line many times. He isn't doing this to make the tattoo hurt worse, but rather to pack more ink into it. Right after the needle first pierces your skin, your body will send phagocytes to that area to start the process of phagocytosis. After a while, your body will realize it can't eat the tattoo and instead build kind of a wall around it, like it would do to any foreign body it can't get rid of on its own.