The process of transferring antibodies or other things into the breast milt is fascinating. New born babies are not very good at generating their own antibodies, and so they rely on the parent to some extent,. And there's a couple of roots through which that can happen. Unlike molecules like caffi, which at diluted levels, can pass kind of passively through the cells and end up in the milk, these antibodies are packaged so receptors on the surface of the cells that line the milk duct will grab them - package them in a 'cunner submarine'