This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
[{'content': 'Which of the following acts as an opsonin? \nSelect and explain. \nA. C3a, B. C3b, C. C5a, D. LTB4', 'role': 'user'}, {'content': 'Answer: B \nExplanation: ANSWER: (B) C3bREF: Robbins 8th ed p. 59See previous question for figure of complement systemsThe following are the basic functions of the complementOpsonization - enhancing phagocytosis of antigensChemotaxis - attracting macrophages and neutrophilsLysis - rupturing membranes of foreign cellsClumping of antigen-bearing agents"The process of coating a particle such as a microbe, to target it for phagocytosis, is called opsonization, and substances that do it are called opsonins"Major opsonins: C3b, IgG, mannose binding lecithin (MBL), fibronectin, fibrinogen and CRP.Membrane attack complex: MAC has a subunit composition of C5b, C6, C7, C8, and polymeric C9Anaphylatoxin: Both C3a and C5a have anaphylatoxin activity', 'role': 'assistant'}]