Erythema nodosum doesn't arrive quietly. One morning you notice warm, tender nodules on your shins — sometimes on the forearms or elsewhere — that are deep red, painful to press, and alarming enough to prompt a doctor visit.

If you've just been told you have Löfgren's syndrome, you've probably already heard the short version: painful red bumps on your shins, swollen ankles, a chest X-ray that surprised everyone by showing enlarged lymph nodes, and a doctor who said some version of "it usually goes away on its own." That last part is true for most people.