An effector T4-lymphocyte uses its TCR and CD4 molecule to bind to a complementary shaped MHC-II molecule with attached peptide epitope on an activated B-lymphocyte. This interaction, along with the binding of co-stimulatory molecules such as CD40 and B7 on the B-lymphocyte with their complementary ligands on the effector T4-lymphocyte triggers the T4-lymphocyte to produce cytokines that enable the activated B-lymphocyte to proliferate, differentiate into antibody-secreting B-lymphocytes and plasma cells, and switch classes of the antibodies being made.