How are the CD4 T cells activated?

CD4 T cells need two signals to become activated; 1. the interaction of the T cell receptor with the antigen on the class II MHC molecules on the APC 2. co-stimulatory signal; binding of B7 molecules on the APC to the CD28 molecules on the T cell when the CD4 T cells are activated they leave the lymph node and go into the blood stream to go to the B cell zones of the lymph node and spleen