Two-signal activation model for B cells Thymus-dependent antigen Helper T celll CD4OL (cD154 CD40 cytokines Signal 1 and signal 2 are not simultaneous Signal 3 But in two steps, signal 2 from Th cellsTwo-signal activation model for B cells Signal 1 and signal 2 are not simultaneous But in two steps, signal 2 from Th cells Signal 3