By co-expressing full-size IgG such as C5-1 (this study) or the chimeric anti-tumor antibody C2B8 (not shown) with protease inhibitors selected based on their inhibitory potency against the host plant’s proteases, we confirm the potential of controlling endogenous proteolysis to increase the accumulation of functional mammalian antibodies at the whole plant scale, and show that leaf developmental stage has a significant impact on the final outcome. The gene discussed is C5; the disease is neoplasm.