A 35-year-old man with a 10-year history of rheumatoid arthritis presents with progressive dyspnea, fever (38.5°C), and a new petechial rash on his lower extremities. Laboratory investigations reveal: hemoglobin 9.2 g/dL, platelet count 85,000/μL, serum creatinine 2.1 mg/dL, and urinalysis showing 3+ proteinuria with RBC casts. Serum complement levels (C3 and C4) are markedly reduced. Skin biopsy shows leukocytoclasia with IgG and C3 deposition in small blood vessel walls. What is the underlying immunological mechanism of this systemic manifestation?