IL21 and infection: We have identified CD4+ T‐cell counts, antigen‐specific T‐cell proliferation and cytokine production (IFN‐γ, IL‐1β, IL‐4, IL‐6, IL‐17, IL‐21 and IL‐31), in response to a variety of opportunistic and vaccine‐preventable pathogens, as having potential to be utilised as biomarkers for infection risk prediction.