File:Activation of T and B cells.png

English: This is a visual depicting how T helper cells and B cells are activated. For T cell activation, there must be binding of the T cell receptor to both the antigen peptide and the MHC class II molecule on an antigen presenting cell (APC). Additionally, there must be binding of the two co-stimulatory molecules (B7 on the APC and CD28 on the T cell). For B cell activation, a pathogen must bind to the IgM and IgD antibodies in order to be internalized and presented on the MHC class II molecule of the B cell. Like T cell activation, there must be binding of the two co-stimulatory molecules (in this case CD40 with CD40L). Once a B cell is activated, it turns into a plasma cell which secretes antibodies.