The antigen receptors on B cells (the B-cell receptor or BCR) and T cells (the T-cell receptor or TCR) are multiprotein complexes made up of clonally variable antigen-binding chains—the heavy and light immunoglobulin chains in the B-cell receptor, and the TCRα and TCRβ chains in the T-cell receptor—that are associated with invariant accessory proteins. The invariant chains are required both for transport of the receptors to the cell surface and, most importantly, for initiating signaling when the receptors bind to an extracellular ligand. Antigen binding to the receptor generates signals that lead ultimately to the activation of nuclear transcription factors that turn on new gene expression and turn off genes typically expressed only in resting cells.