You can bend a string using any finger, what finger you use will be determined by what finger you play before or after the bend.
FIRST FINGER BEND: listen to the audio example and look at the tab of this riff below and you can see and hear that it would be impractical to play it any other way. This is because of the 5 to 7 hammer on (H.O.) with the first two notes. When using the first finger in this example you can either bend up or down (I prefer up).
This riff is using the A Minor Scale - Pattern 1
Bending using your 1st finger is difficult because you can't get a finger behind it to help strengthen the bend.