Ig-isotype switching to IgE requires interleukin-4 and a contact-depen
dent T-cell signal delivered by the ligand for CD40. The recent discov
ery that defects in the gene encoding the CD40 ligand are the underlyi
ng cause of the X-linked hyper IgM syndrome highlights the role of CD4
0 and its ligand in Ig-isotype switching.