A new way to keep immune cells in check

To avoid being attacked by the immune system, normal cells carry proteins on their surfaces that mark them as "self." Until now, researchers have identified only one type of self marker: so-called class I major histocompatibility complex proteins. Now, on page 2051, a team reports that macrophages, the immune system's scavenger cells, recognize a different inhibitory signal--a protein called CD47. The findings also shed light on the role of CD47, a surface protein present on basically every cell type--and long a molecule in search of a function.