Blood banks don't test for lupus or Sjogren's. It is because of the drugs and condition of your red (microcytic/small or anemic) and white cells that you may be rejected. I worked for many years in a blood bank. The blood is screened for HIV and most hepatitis, along with CMV. The blood can't possibly be tested for EVERYTHING!
Two interesting, and very rare, cases come to mind, many years ago. A hospital director was given two units of blood, and died of malaria from the transfusion. Even though the donor had not traveled out of the country in the previous three years (As asked on the screening application then) he had been carrying the dormant malarial parasite for many years, and it was transmitted to the patient!
Not a transfusion story, but, a park ranger had died, and he was an organ donor. His corneas went to a patient, who dies of rabies! The ranger had contracted the rabies virus (which has an affinity for corneal tissue) and passed it on to the cornea recipient.
Anyway, blood banks do a heck of a job keeping the blood supply as safe as possible!