Imprinting equips the immune system with a memory of an invader that helps it prepare to do battle again. The key players are memory B-cells which are generated in lymph nodes during the body's first exposure to a virus. These cells then keep watch in the bloodstream for the same foe, ready to develop into plasma cells that then churn out antibodies. Imprinting was first observed in 1947 by Jonas Sulk and Thomas Francis, the developers of the first flu vaccine - together with another scientist, Joseph Quilligan. They found people who had previously had flu and were then vaccinated against the current circulating strain produced antibodies against the first strain they had encountered. This gave the phenomenon the