T helper cell
Complete stimulation of T helper cells requires the B7 molecule present on the antigen presenting cell to bind with CD28 molecule present on the T cell surface (in close proximity with the T cell receptor). Likewise, a second interaction between the CD40 ligand or CD154 (CD40L) present on T cell surface and CD40 present on B cell surface, is also necessary. The same interactions that stimulate the T helper cell also stimulate the B cell, hence the term "costimulation". The entire mechanism ensures that an activated T cell only stimulates a B cell that recognizes the antigen containing the "same" epitope as recognized by the T cell receptor of the "costimulating" T helper cell. The B cell gets stimulated, apart from the direct costimulation, by certain growth factors, viz., interleukins 2, 4, 5, and 6 in a paracrine fashion. These factors are usually produced by the newly activated T helper cell. However, this activation occurs only after the B cell receptor present on a memory or a naive B cell itself would have bound to the corresponding epitope, without which the initiating steps of phagocytosis and antigen processing would not have occurred.