Erythrocytes were red blood cells, the most common type of blood cell and the principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system.
The dikironium cloud creature was attracted to red blood cells. (TOS: "Obsession")
In 2374, Seven of Nine experienced reduced levels of erythrocytes in her blood after going fifty-eight hours without regenerating in her alcove. (VOY: "Hunters")