The vertebrate immune response involves two major classes of effector cells: T lymphocytes and B lymphocytes. In recent years, the molecules involved in antigen recognition and cell-cell interactions of T and B lymphocytes have been defined at the molecular level. One of the remarkable results of this analysis has been the realization that many of the cell-surface molecules involved in the immune response demonstrate structural similarities at the protein and gene levels. While the surface molecules of T and B cells are functionally diverse, similarities in their structure have suggested that they may have been derived from a single evolutionary precursor.