The CD40-CD40 ligand (CD40L) system is of pivotal importance in the immune
response via interactions between T cells and antigen-presenting cells. Rec
ent reports have demonstrated that CD40 triggering improves tumour immunoge
nicity. Nonetheless, as discussed here, the implications of the CD40-CD40L
system in host-tumour interactions are pleiotropic and should be evaluated
carefully before targeting CD40 for antitumour strategies.