Some people end up feeling crappy after a flu shot, some don’t. Researchers have found a link between complaints of short-term aches and fever after a flu injection and inherent elevated levels of a type of immune cell in those people’s bodies (Nat. Immunol. 2016, DOI: 10.1038/ni.3328). This unexpected immunological discovery, from an international team led by Adrian C. Hayday of King’s College London, carries important implications for understanding postimmunization adverse reactions. The immune cells, known as transitional B cells, produce antibodies and are also associated with some autoimmune diseases, suggesting that people who experience adverse reactions to flu shots may be more at risk for developing autoimmune diseases later in life. The new knowledge could also be important for cancer patients receiving immunotherapy, Hayday tells C&EN. The team injected 178 healthy volunteers with the UK H1N1 Pandremix vaccine, which protects against the swine flu responsible for an outbreak in 2009. In addition to the B-cell discovery, the researchers also gained insight into the role of vaccine adjuvants. Many vaccines, including Pandremix, contain adjuvants to boost efficacy, but their mechanisms of action have remained unclear. Within 24 hours of receiving the H1N1 vaccine, the volunteers’ bodies produced a burst of lymphocytes, a process that may have been boosted by the adjuvant.