wat if B cells did not exist?

B cells though are necessary for the humoral immune response, they do pose lot of problems for a person to design a peptide vaccine where in determination of a B cell epitope is highly crucial as they are not present as linear sequences.....due to their conformational diversity they complicate the task of designing peptide vaccines..well even without B cells one can induce cell mediated immune response with other APC, dendritic cells,macrophages,epithelial cells.when CTL's and CD4+Tcells can effectively protect the host from pathogens why should one need B cells?????