CD28 and influenza: The stimulations are classified into 4 categories: microbial (Bacillus Calmette-Guérin (BCG), Escherichia coli (E. coli), lipopolysaccharide (LPS) and Candida albicans (C. albicans)) and viral (influenza and polyinosinic–polycytidylic acid (poly I:C)) agents, which are predominantly recognized by receptors on innate immune cells; T cell activators (Staphylococcus aureus enterotoxin B superantigen (SEB) and anti-CD3 and anti-CD28 antibodies (anti-CD3 + CD28)), which induce adaptive immune responses; and cytokines (TNF, IL-1β and IFNγ).