Engineers can defuse both friendly and enemy Claymores and AT mines.
They can't however defuse C4, which is a pity actually.
(Probably because they don't carry pliers to cut the red wire. Or was it the green one? Oh noes! )
A lot of times the red skull symbol above friendly AT mines disappears for some reason and I can't distinguish them from enemy ones.
I then have to defuse all of them and plant my own to be sure.