Many of the costimulatory molecules thought to be essential for T-cell
activation have now been identified. The CD40-CD40 ligand interaction
is one such receptor/counter-receptor pair that has been shown to be
important in B- and T-cell cognate interactions. Here Iqbal Grewal and
Richard Flavell examine recent data from studies using gene knockout
techniques that have helped provide a better understanding of CD40L's
role in vivo, in regulation of the immune response.