CD40 and its ligand in the regulation of humoral immunity.

T cells provide help for B cell antibody production by cell-cell contact and by soluble factors. CD40L is the predominant protein induced on activated T cells which constitutes contact-dependent help, and lack of CD40L or blocking CD40-CD40L interactions leads to severely impaired antibody production. In addition to CD40-CD40L, B and T cells express costimulatory, accessory molecules which amplify T and B cell function and allow for reciprocal dialogues during antigen presentation. Interactions between costimulatory counter-receptors can determine lymphocyte activation or nonresponsiveness, and provides a means for regulating self-tolerance.