Dendritic cells, the microbe-sensing alarms of the immune system, can send out a “red alert” to stimulate immunity, or a “calm down” message that tones down excessive immunity that might damage the host. The “calm down” message makes use of vitamin A, providing an explanation for the link between vitamin A deficiency and autoimmune diseases. Bacteria and viruses that cause chronic infections, such as tuberculosis, hepatitis C and HIV, may have evolved strategies that skew this balance of signals in their favor.