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A type of immune cell called a CD8 T cell, which usually kills disease-causing agents, has been found instead to suppress self-reactive immune cells, thereby offering protection against an autoimmune disease in mice.
The immune system has evolved complex mechanisms that allow rapid and destructive responses to microbial intruders while sparing the host’s own tissues. Regulation of this delicate balance depends mainly on the immune system’s two major types of T cell, which are distinguished by the protein — either CD4 or CD8 — that is expressed on their surface. They are called CD4 T cells and CD8 T cells, respectively. The task of CD8 T cells has generally been assumed to be to kill cells infected with microbial invaders and to destroy foreign or abnormal cells. However, writing in Nature, Saligrama et al.1 report another role: CD8 T cells can inhibit self-reactive CD4 T cells and quell autoimmune disease in a mouse model of multiple sclerosis.