Why can't monoclonal antibodies prepared by mouse abdominal cavity be directly used for human treatment? How could I?

Mouse monoclonal antibody belongs to mouse origin. If it is directly used in human body, human immunity will produce serious rejection, cause adverse reactions and even death. Therefore, antibodies of non-human origin cannot be directly used in human body. You can replace the FC segment of mouse antibody with the FC segment of human antibody, and you can escape, but this will also lead to the weakening of antibody killing ability, because the FC segment can not be combined with some complements and so on? Therefore, there are drawbacks, and that's what humanized antibody is doing now. Of course, there are also smaller ones that are made into single chains or smaller ones, only taking the CDR region, and so on. I hope my answer is helpful to you.