C4B and infection: The complement system is essential for eliminating pathogens and infected cells via phagocytosis, cytolysis, and modulation of cellular immune responses.[13] For example, C4B covalently binds to pathogens like Staphylococcus aureus, enhancing their opsonization and clearance.[14] However, excessive complement activation during infection can lead to dysregulated thrombosis, exacerbated inflammation, tissue damage, and irreversible organ dysfunction.[13] In BD patients, the significantly elevated C4B levels observed here may therefore contribute to both pathogenic mechanisms and tissue injury.