    fentence 1: What is a ligand onthe cell  surface’ that is upregulated - after  helper T cell activation?.    fentence.2:'In addition; helper. T: cell  activation ‘causey. an’ upregulation ‘of  molecules: expresed ‘on the T.: cell's  surface; . sith. as--CD40 ligand. (alo  called: .CD154),- which provide extra  stimulatory signals typically required  to activate antibody-producing B cells.