The invention concerns immunotoxins and methods of making the immunotoxins. Targeting of the immunotoxins occurs via an antibody that is specific to T cells. This allows the specific ablation of malignant T cells and resting T cells. The transient ablation of resting T cells can "reset" the immune system by accentuating tolerizing responses. As a result, the immunotoxin can be used to treat autoimmune disease, malignant T cell-related cancers, and graft-versus-host disease. The toxin portion of the immunotoxin is engineered to maintain bioactivity when produced in yeast, specifically Pichia pastoris. This system allows the production of dimeric antibody fragments with increased binding affinity and potency.