One application for immunization of pregnant or lactating animals comes from the arena of mastitis control in cattle. Mastitis is the major disease in dairy cattle and most often is caused by intramammary infection [83, 84] . Vaccination of cattle against mastitis-causing pathogens has been an area of study for many years [85, 86] . Optimization of immunization schedules continues to be investigated [87] . Effective vaccines against mastitis-causing pathogens can increase antigen-specific immunoglobulins in the serum, which in turn can be increased in the mammary secretions. In the case of the J5 E. coli bacterin vaccine, the immunization also may be causing the mammary gland to become hyper-responsive to bacterial challenge [88] , reminding us that enhancement of antigen-specific antibodies in the milk is not the only mechanism by which the vaccine may be having its effect.