If you look at the histogram in the Levels Properties you can see that it has very low contrast and no content anywhere near full black and full white.

If we adjust the input sliders to give the image a full tonal range, it will maximise contrast, but stretch out the original tonal range to fill all 256 steps (assuming 8 bit).

If we now add a second levels adjustment layer, but not to make furth adjustments, but just see the resulting histogram we see it is full of holes.

If this was a landscape with subtle blue to lighter blue sky gradient, then that gappy histogram might mean banding if you stretch the tonal range globally.  There was an interesting video on the recent Photoshop Summit 8 from Blake Rudis which suggested using Blend If to target/preclude specific color ranges.  I’d never thought of this before, and he gave a convincing demonstration.  That video is no longer available, but he has a YouTube channel.  This video might cover it.

Taking a guess without seeing a sample of your western blot, unless you’ve used a selection tool (e.g., Marquee) to select a portion of the image, the change is global, not local.

Adjust color and tone with Levels and Curves eyedroppers