 Firstly, B cell receptors, which are just IgM and IgD antibodies specific to the particular B cell, must bind to the antigen which then results in internal processing so that it is presented on the MHC class II molecules of the B cell.Once this happens a T helper cell which is able to identify the antigen bound to the MHC interacts with its co-stimulatory molecule and activates the B cell.As a result, the B cell becomes a plasma cell which secretes antibodies that act as an opsonin against invaders.