Fluid Accumulation

In a joint that is affected by arthritis, the body tries to protect the joint by creating a cushion of fluid, much like a blister that you might suffer when wearing new shoes. Its our bodies way of trying to protect the area being aggravated by friction of some kind.
The same kind of fluid accumulation is also what happens when our body tries to protect you from harmful chemicals, like we see happen when we experience an allergy for nail coatings. Fluid builds up under the skin causing little blisters, these blisters are natural cushions that protect our epidermis layer from more (chemical) invasion.