A T4-lymphocyte uses its TCR and CD4 to recognize a MHC-II molecule with attached peptide being presented by the APC. This interaction, along with secondary interactions such as B7, CD40, LFA-3, and ICAM-1 molecules on the APC binding to corresponding ligand molecules (CD28, CD40L, CD2, and LFA-1) on the naive T4-lymphocyte, results in activation of the T4-helper cell and its differentiation into effector T4-lymphocytes such as Th1 and Th2 cells.