WHAT DOES BONE MARROW DO? Bone marrow makes millions of blood cells every second to replace old, worn-out blood cells, which the body destroys. There are two types of marrow: red and yellow. Red marrow makes blood cells. Yellow marrow is mainly a fat store, but it can turn into red marrow if the body needs extra blood cells. At birth, nearly all bone marrow is red. During the teens, much of it turns into yellow bone marrow.