Ripped comment:
The title is a little misleading. He isn’t asking for immunity from any crimes he may have committed, he is saying he wants immunity on the testimony he will give. In other words, whatever testimony he gives to congress will not be admissible in a future criminal prosecution. So he still could be prosecuted, but they couldn’t use this testimony against him.
He will tell all to save his skin
http://www.investmentwatchblog.com/comey-tweet-begging-mccabe-for-mercy/