How can B cell mature into plasma cell?

A naive B cell (has not encountered a pathogen that it can bind to) resides in your secondary lymph system. It expresses mainly IgM and IgD antibodies on its cell surface. A bacteria that enters your blood system will pass through your lymphatic system. When the naive B cell that recognizes a specific bacteria with it's antibody it will engulf it and present its peptides on MHC class II. However, this is generally insufficient to activate the B cell. It requires the help from a CD 4 T cell, more specifically Th2. Th2 cell that was activated from a dendritic cell will activate the B cell mainly with cytokines IL-4, IL-5, IL-10. This will fully activate the B cell into proliferation into plasma B cells and memory B cells. The plasma cell will undergo somatic hypermutation to increase specificity and isotype switching (IgA, IgE or IgG).