E-Mail IMAGE: A newly developed B cell (green in middle of image) migrates from the bone marrow of the skull, where a cluster of other new B cells remain, then through the... view more Credit: Simone Brioschi The immune system is the brain's best frenemy. It protects the brain from infection and helps injured tissues heal, but it also causes autoimmune diseases and creates inflammation that drives neurodegeneration. Two new studies in mice suggest that the double-edged nature of the relationship between the immune system and the brain may come down to the origins of the immune cells that patrol the meninges, the tissues that surround the brain and spinal cord. In complementary studies published June 3 in the journal