The fate of tattoos

I'm taking Immunology this sem with labmate Angela and boyfriend Steve (and my poor engineer is going to die of acronym bombardment, he's not used to us biologists). So far we're going over the basics...classes of immune cells, the difference between adaptive and innate immunity and whatnot.

If you're a layperson reading this and want to know a bit about immunology I recommend Lennart Nilsson's dramatically beautiful The Body Victorious - you can see some pics from it here. He's most famous for A Child Is Born, which is the one that has all those neat pictures of fetuses.

Anyway, the prof today was talking about macrophages, which are big cells that eat things (you can basically figure it out from the name if you know Greek). One slide was about tattoos, and it finally answered the question I've been wondering about, which is why don't the macrophages eat up the tattoo ink and carry it away?
(Slide copyright Gary A. Splitter)

Actually, I like Sgt. Colon's explanation better (from Terry Pratchett's Jingo, which I think should be recommended reading for all citizens of any nation with a military):

"Sarge," said Nobby, as they looked out at the wonders of the deep.
"Yes, Nobby?"
"You know they say every tiny part of your body is replaced every seven years?"
"A well-known fact," said Sergeant Colon.
"Right. So...I've got a tattoo on my arm, right? Had it done eight years ago. So...how come it's still there?"
Giant seaweeds winnowed the gloom.
"Interesting point," quavered Colon. "Er..."
"I mean, okay, new tiny bits of skin float in, but that means it ought to be all new and pink by now."
A fish with a nose like a saw swam past.
In the middle of all of his other fears, Sergeant Colon tried to think fast.
"What happens," he said, "is that all the blue skin bits are replaced by other blue skin bits. Off'f other peoople's tattoos."
"So...I've got other people's tattoos now?"
"Er...yes."
"Amazing. 'cos it still looks like mine. 's got the crossed daggers and 'WUM.'"
"Wum?"
"It was gonna be 'Mum' but I passed out and Needle Ned didn't notice I was upside down."