Nephrotic syndrome occurs when the filters in the kidney leak an excessive amount of protein.
The level of protein in the blood then falls and this allows fluid to leak across very small blood vessels into the tissues. Swelling around the eyes, abdomen and legs is a consequence of this process. Protein in the body is normally made by the liver and the liver then responds by producing extra protein. Most cases of nephrotic syndrome are caused by changes in the immune system. Certain types of white cell may produce substances that act on the filters in the kidney to produce a change in the filter permeability or leakiness.