Plants are capable of synthesizing and assembling virtually every kind of antibody molecule, ranging from the smallest antigen-binding domains and fragments, to full-length, and even multimeric, antibodies. A number of plant hosts can be used, and because this is a versatile expression system that can be scaled-up to agricultural proportions, a cheap and plentiful supply of antibodies could be made available. Immunotherapy is one of the many potential uses for bulk quantities of antibody. In particular, passive immunotherapy of mucosal surfaces may be possible, because functional secretory antibodies can be engineered in plants.