A memory of microbes may help the immune system to fight pathogens that it has never encountered before.

Mark Davis and his colleagues at Stanford University in California examined white blood cells called T cells that carry the CD4 antigen — which quickly rouse other cells into launching a robust immune attack — in blood samples of 26 healthy adults. Although all the blood donors tested negative for HIV, the researchers found unexpectedly high levels of the T cells that recognized the virus and were primed to launch an attack. When they analysed umbilical-cord blood from newborns, however, they found that these 'memory' cells were absent, which may explain why young children are so susceptible to infection. The researchers also showed that vaccination against flu activated cells that recognize proteins not just from flu, but other bacterial species.

These findings may help to explain why vaccinations against one disease can offer protection against other infections.

Immunity http://dx.doi.org/10.1016/j.immuni.2012.10.021 (2013)