'''Killed inactivated organism or toxin (toxoid)''' are useful where virulent and toxic organisms cannot be used as vaccines as they would cause disease. Organisms can be killed using radiation or chemicals so that they still possess the antigens to stimulate an immune response, but the organisms are unable to replicate inside the host. Alternatively, toxins are inactivated to produce a toxoid which will still have the antigens needed to produce an immune response but will not be harmful to the host. Two doses are required to [[B_cell_differentiation#Primary_T_Cell_Dependent_Response|'''prime''']] the immune system initially, and then induce an immunoligical [[B_cell_differentiation#Secondary_T_Cell_Dependent_Response|'''memory''']] of the disease casuing organism. | '''Killed inactivated organism or toxin (toxoid)''' are useful where virulent and toxic organisms cannot be used as vaccines as they would cause disease. Organisms can be killed using radiation or chemicals so that they still possess the antigens to stimulate an immune response, but the organisms are unable to replicate inside the host. Alternatively, toxins are inactivated to produce a toxoid which will still have the antigens needed to produce an immune response but will not be harmful to the host. Two doses are required to [[B_cell_differentiation#Primary_T_Cell_Dependent_Response|'''prime''']] the immune system initially, and then induce an immunoligical [[B_cell_differentiation#Secondary_T_Cell_Dependent_Response|'''memory''']] of the disease casuing organism. |