There is an increase in circulating B cells, including mature (CD19+ CD5+) and precursor (CD19+ CD10+) cells, in infants with RSV LRTI, and CD20+ B cells and IgM+, IgG+, and IgA+ plasma cells are prominent in postmortem lung tissue from infants with fatal RSV bronchiolitis (106–108). This evidence concerns the gene CD40LG and respiratory syncytial virus bronchiolitis.