If cysts are filled with fluid, what are nodules filled with? Acne Question.

Cysts Nodules. In the skin, a nodule would be a solid, firm structure that usually would be due to scarring from acne, though it could be a solid tissue such as a cancer in other settings.
Here's the facts. Cysts are by definition filled with fluid, and it's actually a misnomer -- acne "cysts" are actually filled with pus and lack the inner epithelium that defines a true cyst. Nodules are inflamed areas without liquid, but are hard and swollen because blood vessels have released protein and a host of inflammatory cells, mostly neutrophils, are rampaging in the area. If bad enough, this becomes pus.