A group of red blood cells arranged together like a roll of coins, usually only noticed on a slide under a microscope. Since red blood cells in a reasonably healthy person should have a mutually repelling membrane charge, this means that something like an inflammatory response or an elevation of liver-synthesized lipids (LDLs and VLDLs) is occurring. Inflammation makes the blood "sticky," and the lipids from the liver lower the charges. Remember, of course, that I am talking about subclinical imbalances...such things as rouleau can accompany some pretty gnarly diseases. Our kind of rouleau can give you a headache or make your hands and feet cold because it's hard to push rolls of coins through little bitty capillaries.