In the effector phase of delivering T cell help for B cell differentiation, proteins induced following helper T cell activation deliver cell contact signals to the B cell that drive its activation and proliferation. Activation induced soluble lymphokines also deliver signals that, in conjunction with the contact signals, drive B cell differentiation and immunoglobulin class switching. Helper T cell activation leads to transient expression of a ligand for the constitutively expressed B cell surface protein, CD40. The CD40 ligand is involved in delivering the T cell induced contact signals. Regulation of synthesis of the CD40 ligand by activated helper T cells contributes to the maintenance of specificity of the antibody response. Knowledge of the molecules involved in delivering T cell help allows a clearer understanding of events regulating antibody affinity maturation, selection and class switching in vivo.