Interstitial nephritis

Interstitial nephritis is inflammation of the kidney, but not of just any part of the kidney. The kidney has glomeruli, the filters where urine begins, tubules, the ducts that carry that urine onward, and the interstitium, the packing tissue between the two. In this disease inflammatory cells invade the interstitium and the tubules, and leave the glomeruli alone. The inflammation is in the packing between the tubes, not in the filters.

In its acute form, the most frequent cause in developed countries is a medication. The cells of the tubules or the interstitium endocytose the drug, meaning they pull it inside, and there it becomes a hapten: a small molecule that on its own does not wake up the immune system, but does once it attaches to a large protein.

What follows, according to current thinking, is a type IV delayed hypersensitivity, an allergic reaction carried out not by antibodies but by T lymphocytes, the defense cells that attack directly. That form comes with an acute drop in kidney function, meaning the kidney stops filtering the way it should.