A 32-year-old woman with systemic lupus erythematosus (SLE) and chronic renal failure manifests rapidly progressive weakness. On physical examination, she appears pale and has slightly yellow sclerae and an enlarged spleen. Blood tests reveal severe anemia and mild, mostly unconjugated, hyperbilirubinemia. Coombs test is positive at 37 C but negative at 0-4 C. This patient developed anemia because of?