Via ScienceDaily: "researchers studied hematopoietic stem cells, which create the cells that comprise the blood and immune system. Understanding when and how these stem cells begin to falter as the years pass may explain why some diseases, such as acute myeloid leukemia, increase in prevalence with age, and also why elderly people tend to be more vulnerable to infections such as colds and the flu. ... We know that immune system function seems to decline with increasing age. This is the first study comparing the function and gene expression profiles of young and old purified, human hematopoietic stem cells, and it tells us that these clinical changes can be traced back to stem cell function. ... Specifically, the researchers found that hematopoietic stem cells from healthy people over age 65 make fewer lymphocytes - cells responsible for mounting an immune response to viruses and bacteria - than stem cells from healthy people between ages 20 and 35. (The cells were isolated from bone marrow samples.) Instead, elderly hematopoietic stem cells, or HSCs, have a tendency to be biased in their production of another type of white blood cell called a myeloid cell. This bias may explain why older people are more likely than younger people to develop myeloid malignancies."