C2 and systemic lupus erythematosus: Systemic lupus erythematosus (SLE) is a complex autoimmune disease in which the complement system plays a dual role: deficiency of early components (C1q, C2, C4) predisposes to disease onset, while overactivation drives inflammation and organ damage, particularly in lupus nephritis and antiphospholipid syndrome [36,37,38].