Transgenic plants represent an alternative to cell culture systems for producing cheap and safe antibodies for diagnostic and therapeutic use. To evaluate the functional properties of a plantibody, we generated transgenic Arabidopsis plants expressing full-length human IgG1 against the Rhesus D antigen, which is responsible for alloimmunization of RhD– mothers carrying an RhD+ fetus. Anti-RhD extracted from plants specifically reacted with RhD+ cells in antiglobulin technique, and elicited a respiratory burst in human peripheral blood mononuclear cells. Plant-derived antibody had equivalent properties to CHO cell-produced anti-RhD antibody, indicating its potential usefulness in diagnostic and therapeutic programs.