People who reported higher levels of side effects had higher levels of expression of genes affiliated with B cells, which make antibodies, both before and after receiving the vaccine. The findings, reported online today in Nature Immunology, are “wholly unexpected,” says immunologist Adrian Hayday of King’s College London, who led the study. As Hayday explains, his team looked at myriad genes expressed in blood samples, most of which were not related to B cells. The researchers also found different gene expression patterns after the flu vaccination in younger and older people. That may be because people over 40 were more likely to have had been exposed to a related swine flu virus, which altered their immune responses.