Grace Park appears briefly in this episode as an X5. She would go onto become "Number Eight", a humanoid Cylon on Battlestar Galactica (2004).
Logan is infected with a designer retrovirus; if he cooperates with Renfro she will give him the antigen. An antigen is a molecule which triggers the immune system to fight whatever has that molecule on it. You give folks antigens when you vaccinate them. Antibodies are molecules the immune system makes to destroy antigens. You can prepare antisera rich in antibodies and give that to someone who would die before their body had time to respond to a vaccine.