    ‘Sentence i: What is a Ligand on the cell surface that}  is upregulated ofter helper T cell activation?    Sentence 2: Helper T cells have @ weaker  @ssociation with the MHC:antigen. complex than|  observed for killer T cells, meaning many receptors|  {around 200-300) on the helper T cet must .de|  bound by on MHCiantigen in order to activate the|  helper cell, while kitler T celts can be activated by|  engagement of a single MHCrantigen motecute.    