This is a very educational rash!
There is a center area with deep, bubbly spots, one of which looks like it broke open and got infected. (Remember that poison ivy rash is an allergic reaction but it can become infected just like any break in your skin.)
But we also see a strange line of smaller bubbles coming downward from the main rash. What is that?
While it might be tempting to think that it is the result of fluid from the bubble leaking and causing more rash, it is probably a result of the original incident: a broken poison ivy vine might have poked right into the leg causing the main rash, but that also left a trail of plant oil running down the leg that created the strange downward rash pattern.