Uh mucosal immunity is one of the most important steps to preventing and clearing infection. Barrier immunity takes care of the bulk of pathogens, complement is then the next biggest protection, and even with the antibody response, mucosal antibodies are the first to be produced. The antibody response takes several days to develop, often too late given the stage of disease. Innate immunity is incredibly important to prevent and constrain the course of disease. Some people can live (tough) lives with total deficiency of antibodies, but not a single human has been born alive without complement for instance. Edit: the same goes for skin infections. The skin is the most important barrier, followed by complement, then peripheral immune cells (mast cells, langherans cells, ...). When bacterial infections reach the blood, you're often pretty fucked.