This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
[{'content': 'Opsonin among the following? \nSelect and explain. \nA. C3a, B. C3b, C. C4a, D. C5a', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Opsonization and phagocytosis: C3b and its cleavage product iC3b (inactive C3b), when fixed to a microbialcell wall, act as opsonins and promote phagocytosis by neutrophils and macrophages, which bear cell surface receptors for the complement fragments.Ref: Robbins and Cotran Pathologic Basis of Disease; 9th edition; Chapter 3; Inflammation and Repair; Page no:89', 'role': 'assistant'}]